AoC 2021, day 12 pt 2 (Perl)

#!/usr/bin/perl

use strict;
use warnings;

my %Graph;

# read in graph
while (<>) {
    my ($a, $b) = m#(start|end|..)-(start|end|..)#;

    push( @{$Graph{$a}}, $b );
    push( @{$Graph{$b}}, $a );
}

sub recurse_path {
    my ($loc, $double, %visit) = @_;
    my $ret = 0;

    # reached end, return 1 way
    return (1) if ($loc eq 'end');

    # handle small cave... set double if second time here
    if ($loc eq lc($loc)) {
        $visit{$loc}++;
        $double = 1 if ($visit{$loc} == 2);
    }

    # recurse to valid neighbours
    foreach my $neigh (@{$Graph{$loc}}) {
        next if ($neigh eq 'start');                    # only once ever
        next if (exists $visit{$neigh} && $double);     # this would be a second double

        $ret += &recurse_path( $neigh, $double, %visit );
    }

    return ($ret);
}

print "Part 2: ", &recurse_path( 'start', 0 ), "\n";