use strict;
use warnings;

my $data1 = <<DATA;
1 a
2 b
3 c
DATA
my $data2 = <<DATA;
4 d
1 x
DATA

my %data2Hash;

open my $infile2, '<', \$data2;
while (<$infile2>) {
    chomp;
    my ($key, $tail) = split ' ', $_, 2;
    $data2Hash{$key} = $tail;
}
close $infile2;

open my $infile1, '<', \$data1;

while (<$infile1>) {
    chomp;
    my ($key, $tail) = split ' ', $_, 2;

    if (exists $data2Hash{$key}) {
        print "Matched $key: $data2Hash{$key}, $tail\n";
    }
}