file-1:                     file-2
1000 - foo                  2000
2000 - bar                  3000
3000 - fubar                .... 
...                         990000 
1000000 - heck

##</code><code>##

use autodie qw(open close);

open my $fip, '<', 'file-1';
open my $fop, '<', 'file-2';

LINE1:
while (my $line1 = <$fip>) {
    my @token = split(/-/, $line1, 2);

    while (my $line2 = <$fop>) {
        chomp $line2;
        if ($token[0] == $line2) {
            print $line1;
            next LINE1;
        }
    }
}

close $fip; 
close $fop;

##</code><code>##

use autodie qw(open close);

open my $fip, '<', 'file-1';
open my $fop, '<', 'file-2';

my %record_id;
while (my $line = <$fop>) {
   chomp $line;
   $record_id{$line} = 1;
}

close $fop;

while (my $line = <$fip>) {
    my @token = split(/-/, $line, 2);
    if ( exist $record_id{$token[0]} ) {
        print $line;
    }
}

close $fip;