#!/usr/bin/perl
use strict;
use warnings;

my %net = (
    '1.2.3.4' => ['1.2.3.5', '1.2.3.7'],    
    '1.2.3.5' => ['1.2.3.4', '1.2.3.6', '1.2.3.7'],
    '1.2.3.6' => ['1.2.3.5', '1.2.3.7'],
    '1.2.3.7' => ['1.2.3.6', '1.2.3.4', '1.2.3.5'],
);
print find_path('1.2.3.4', '1.2.3.6', %net);

sub find_path {
    my ($beg, $end, %net) = @_;
    my (%seen, %stop, @work);
    for (@{$net{$end}}) {
        return "$beg=>$end" if $_ eq $beg;
        $stop{$_} = 1;
    }
    push @work, {key => $_, path => "$beg=>$_"} for @{$net{$beg}};   
    while (@work) {
        my $node = pop @work;
        next if $seen{$node->{key}}++;
        return "$node->{path}=>$end" if $stop{$node->{key}};
        push @work, {key => $_, path => "$node->{path}=>$_"} for @{$net{$node->{key}}};
    }
    return 'path not found';
}