use strict;
use warnings;
use feature qw( say );

sub _write {
   my ( $err, $num ) = @_;
   say "$err, $num";
}

my %data;

$data{error} = "";
_write( $data{error} || 0, 42 );

$data{error} = "foo bar";
_write( $data{error} || 0, 42 );

$data{error} = "foo baz";
_write( $data{error} or 0, 42 );  # <-- fails

__END__

0, 42
foo bar, 42
Use of uninitialized value $num in concatenation (.) or string at i line 7.
foo baz,