If the grid method is under your control, then you can alleviate the need for the temporary var $item, and the conditional by arranging for it to return the empty list rather than undef when it has nothing to return.

#! 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";

__END__
[20:55:29.43] P:\test>junk
1 2 3 4
1 2 3 4 12345
1 2 3 4 12345
[download]

As you can see, pushing the empty list onto the array has no effect upon it, so you save the need to test the return and a temporary variable.

If it isn't under your control and you need to use the temp var and a test, your second working form is probably slightly preferable to the first as at least the temp var goes away, and should you need to do the same thing further down you can use the same temp var name and code form without conflict.

#! perl -slw
use strict;

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

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

if( my $item = grid( 1 ) ) {
    push @array, $item;
}

if( my $item = grid( 1 ) ) {
    push @array, $item;
}

print "@array";

__END__
[21:01:53.53] P:\test>junk
1 2 3 4
1 2 3 4 12345 12345
[download]