my proposed solutions for challenge-015, p5 1 & 2

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diff --git a/challenge-015/daniel-mantovani/perl5/ch-1.pl b/challenge-015/daniel-mantovani/perl5/ch-1.pl
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+++ b/challenge-015/daniel-mantovani/perl5/ch-1.pl
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+# Write a script to generate first 10 strong and weak prime numbers.
+
+#   For example, the nth prime number is represented by p(n).
+
+#   p(1) = 2
+#   p(2) = 3
+#   p(3) = 5
+#   p(4) = 7
+#   p(5) = 11
+
+#   Strong Prime number p(n) when p(n) > [ p(n-1) + p(n+1) ] / 2
+#   Weak   Prime number p(n) when p(n) < [ p(n-1) + p(n+1) ] / 2
+
+use strict;
+use warnings;
+use v5.10;
+
+# we sart by using the same prime test used to solve challenge 12 - 1
+# (the regexp prime tester)
+#
+
+sub is_prime {
+    return ( 7 x shift ) !~ /^(77+)\1+$/;
+}
+
+# we will allow changing the size of the lists, and use 10 as the default
+my $list_length = shift // 10;    #
+
+# we define array for all primes, populating with the first two,
+# so for n=2, p(n-1) would be 2, p(n) is 3, and p(n+1) is yet
+# to be calculated
+
+my @primes = ( 2, 3 );
+
+# we also define arrays for strong and weak primes
+my ( @strong, @weak );
+
+# we will loop until we get at least required amount for both strong and weak primes
+while ( @strong < $list_length || @weak < 10 ) {
+    my $p_n   = $primes[-1];    # this is p(n)
+    my $p_nmm = $primes[-2];    # this is p(n-1)
+    my $p_npp = $p_n + 1;       # and we will calculate p(n+1)
+
+    # get next prime
+    $p_npp++ while !is_prime($p_npp);
+    push @primes, $p_npp;
+    my $check = ( $p_nmm + $p_npp ) / 2;
+    push @strong, $p_n if $p_n > $check;
+    push @weak,   $p_n if $p_n < $check;
+}
+
+# now we just print results
+say "First $list_length strong primes: " . join ', ', @strong;
+say "First $list_length weak primes: " . join ', ',   @weak;
diff --git a/challenge-015/daniel-mantovani/perl5/ch-2.pl b/challenge-015/daniel-mantovani/perl5/ch-2.pl
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+# Write a script to implement Vigenère cipher.
+# The script should be able encode and decode.
+# Checkout wiki page for more information.
+
+use strict;
+use warnings;
+use v5.10;
+
+# Vigenère cipher works only with ascii uppercase letters, no
+# spaces, control chars, etc
+#
+
+# we will receive 3 arguments, a direction that could be E for encrypt
+# or D for decript, a keyword and the string to encode / decode:
+
+my ( $dir, $key, $string ) = @ARGV[ 0 .. 2 ];
+
+die
+"Usage: $0 <E/D> <keyword> <string>\nwith both keyword and string a sequence of uppercase plain english letters"
+  unless $dir
+  && $dir =~ /^(e|d)$/i
+  && $key
+  && $key =~ /^([A-Z])+$/
+  && $string
+  && $string =~ /^([A-Z])+$/;
+
+# we start by defining two arrays, for the keyword and the text
+# the arrays will have integers from 0 to 25, representing upper
+# case letters from A to Z
+
+my @keyword = map { ord($_) - 65 } split '', $key;
+my @text    = map { ord($_) - 65 } split '', $string;
+
+# we should repeat keyword letters until we match text size, but
+# we are not gonna do that, we just use module arithmetic to get
+# the same effect, using keyword length as the module
+
+my @result;
+
+# encrypting is just adding text and keyword values, and
+# decrypting would be the opposite, just substract keyword
+# values from text
+# so we define a sign variable to take care of that
+
+my $sign = 1;    # encryption case
+$sign = -1 if $dir =~ /d/i;
+
+# now we do encryption or decryption letter by letter
+for my $i ( 0 .. $#text ) {
+    $result[$i] = $text[$i] + $sign * $keyword[ $i % @keyword ];
+}
+
+# and now we just print the result, after reconverting it
+# to letters one by one
+
+say join( '', map { chr( ( $_ % 26 ) + 65 ) } @result );
+
+# example usage (examples from wikipedia page):
+# ✘ $> perl ch-2.pl E LEMON ATTACKATDAWN
+# LXFOPVEFRNHR
+#  $> perl ch-2.pl D LEMON LXFOPVEFRNHR
+# ATTACKATDAWN
+#  $>