I'd go for the first version, with the temporary variable, for the exact reason you mention: the second version has an extra call to the subroutine. The extra memory overhead shouldn't be much of a problem. I don't think you're dealing with gigabites of data here at once.

Another reason the first version is preferable because the call to the subroutine in the if clause may have side effects that will affect the value that will be pushed onto the array. The following script demonstrates what I mean.

Update: there was a stupid little error in the script. Fixed.

use strict;
use warnings;
use subs qw(
    version1
    version2
);

my @delete = qw(bar baz);
my %hash = (foo => 1, bar => 2, baz => 3);
print "Keys in the hash: ", join(", ", keys %hash), "\n";

print "Version 1 reports ", version1, " as the sum of values deleted f
+rom the hash.\n";
print "Keys in the hash after version 1: ", join(", ", keys %hash), "\
+n";

print "\n";

%hash = (foo => 1, bar => 2, baz => 3);
print "Keys in the hash: ", join(", ", keys %hash), "\n";
print "Version 2 reports ", version2, " as the sum of values deleted f
+rom the hash.\n";
print "Keys in the hash after version 2: ", join(", ", keys %hash), "\
+n";


sub version1 {
    my $num_deleted = 0;
    for (@delete) {
        my $deleted = delete $hash{$_};
        $num_deleted += $deleted if $deleted;
    }
    return $num_deleted
}

sub version2 {
    my $num_deleted = 0;
    for (@delete) {
        $num_deleted += delete $hash{$_} if delete $hash{$_}
    }
    return $num_deleted
}

__END__
Keys in the hash: bar, baz, foo
Version 1 reports 5 as the sum of values deleted from the has.
Keys in the hash after version 1: foo

Keys in the hash: bar, baz, foo
Use of uninitialized value in addition (+) at G:\y.pl line 35.
Use of uninitialized value in addition (+) at G:\y.pl line 35.
Version 2 reports 0 as the sum of values deleted from the hash.
Keys in the hash after version 2: foo
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