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

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

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