#!/usr/bin/env perl
use Modern::Perl;

my @p;

open my $fd2, '<', 'INPUT_FILE2.TXT' or die $!;
while(<$fd2>){
    next if /#/ or ! /\w/;
    my( $p, $n ) = split;
    push @{$p[$n]}, $p;
}
close $fd2;

open my $fd1, '<', 'INPUT_FILE1.TXT' or die $!;
while(<$fd1>){
    next if /#/ or ! /\w/;
    my( $r, $n ) = split;
    say "reactant: $r, product: $_" for @{$p[$n]};
}
close $fd1;