As written, you subroutine can be called without parameters. Note that write is not a good subroutine name, though, as write already has a meaning in Perl.

On the other hand, you can write the subroutine in a way it expects a parameter:

sub mywrite {
    my $candidate = shift;
    print "$candidate->{firstname} $candidate->{surname}:$candidate->{
+id}\n";
}
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This still look wrong, though. What's the relation between $guest and $kandydatkonkursu?