Your problem lies in the fact that you're calling sendmail as a method.

$mail->sendmail({to=>'koszta@seznam.cz', from=>'admin@server.com', dat
+a=>'hi from server',
+ subject=>'msg body'});
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Either, call it as a function if that is what's appropriate

sendmail({to=>'koszta@seznam.cz', from=>'admin@server.com', data=>'hi 
+from server',
+ subject=>'msg body'});
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Or remember to pull the object reference first.

sub sendmail {
   my $self = shift;
   my $param = shift;
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