use strict;
use warnings;
use Other::Library qw/ savemail /;
...

##</code><code>##

{
       $_ =~ m/([^:]*):(\S*)/;
       $username = $1;
       $password = $2;
       &checkdomain($username);
    }

##</code><code>##

open my $FH, '<', $mails or die "Nie moge otworzyc pierwszego pliku";
while ( <$FH> ) {
  chomp;
  my ($username, $password) = split /:\s*/;
  checkdomain($username);
}