Just to propose a TMTOWTDI solution that nobody else suggested yet..

Variables passed to a subroutine are automatically aliased to the @_ array, so if you modify them, you modify the original variables passed in:

use strict;

my $X = 1;
my $Y = 2;
my $Z = 3;

print "before: X=$X, Y=$Y, Z=$Z\n";
modifyvars($X, $Y, $Z);
print "after: X=$X, Y=$Y, Z=$Z\n";

sub modifyvars {

    $_[0] = 2;
    $_[1] = 3;
    $_[2] = 4;

}
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Though this isn't too easy to read. I would have to agree with Zaxo -- if your version of Perl is new enough, prototypes make this cleaner to look at.