perl code for challenge 79 task 2

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diff --git a/challenge-079/walt-mankowski/perl/ch-2.pl b/challenge-079/walt-mankowski/perl/ch-2.pl
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+#!/usr/bin/env perl
+use strict;
+use warnings;
+use feature qw(:5.32);
+use experimental qw(signatures);
+use List::Util qw(max);
+
+# TASK #2 › Trapped Rain Water
+# Submitted by: Mohammad S Anwar
+#
+# You are given an array of positive numbers @N.
+#
+# Write a script to represent it as Histogram Chart and find out how
+# much water it can trap.
+
+my @a = @ARGV;
+
+# build the histogram
+my $rows = max(@a);
+my $cols = @a;
+my @hist;
+for my $col (0..$#a) {
+    for my $row (0..$rows-1) {
+        $hist[$row][$col] = $row < $a[$col] ? 1 : 0;
+    }
+}
+
+my $units = 0;
+for my $row (1..$rows-1) {
+    for my $col (1..$cols-1) {
+        $units++ if trapped($row, $col, $cols, @hist);
+    }
+}
+
+say $units;
+
+sub trapped($row, $col, $max_col, @hist) {
+    # check if we're at a wall
+    return 0 if $hist[$row][$col];
+
+    # look for left wall
+    my $left = 0;
+    for (my $c = $col-1; $c >= 0 && !$left; $c--) {
+        $left = 1 if $hist[$row][$c];
+    }
+
+    # look for right wall
+    my $right = 0;
+    for (my $c = $col+1; $c < $max_col && !$right; $c++) {
+        $right = 1 if $hist[$row][$c];
+    }
+
+    return $left && $right;
+}