use strict;
use warnings;
use Data::Dumper;

open ( my $fh2, '<', 'file2.txt') or die ("cannot open file1: $!");
my %test;
while(my $line = <$fh2> ){
    chomp $line;
    my( $part1, $part2) = split(/_/, $line);
       $test{$part1} = $part2;
   }
close $fh2;
open ( my $fh1, '<', 'file1.txt') or die ( "Cannot open the file2: $!");
my @file1;
while (my $line = <$fh1>) {
    chomp $line;
    push @file1, split /,/, $line;
}
close $fh1;
open(my $wh, '>', 'file3.txt') or die ( "cannot open file2 to write: $!");

for my $file1 (@file1) {
    if (exists $test{$file1}) {
        print $wh join("_", ($file1, $test{$file1}."\n" ));
    }
}
    
close $wh;