Making grid() return the empty list will do the right thing in a scalar context too.

#! perl -slw
use strict;

sub grid{ 
    return $_[0] ? 12345 : ();
}

my @array = 1 .. 4;
print "@array";

push @array, grid( 1 );
print "@array";

push @array, grid( 0 );
print "@array";

my $item = grid( 0 );

print 'The null list does the right thing in a scalar context'
    if not defined $item;

__END__
[ 7:10:52.28] P:\test>junk
1 2 3 4
1 2 3 4 12345
1 2 3 4 12345
The null list does the right thing in a scalar context
[download]

It avoids the temp var and the need to code the conditional at every use by moving the replicated code inside the subroutine where it belongs.

You could also use wantarray to test the context and return wantarray ? () : undef; but that's completely unnecessary also.

The very simplest thing to do when there is nothing to return is just return:

sub grid {
    my( $x, $y ) = @_;
    if( outOfRange( $x) 
    or outOfRange( $y ) {
        return;
    else {
        # determine return value...
        return $rv;
    }
}
[download]

When Perl encounters the empty return, it determines the context and return the appropriate value for you. Some people don't like that though.