use strict;
use warnings;

use Data::Dumper;

testsub( my $ref1, 'hi' );
print Dumper( $ref1 );
# output: $VAR1 = undef;

my $ref2;
testsub( $ref2, 'hello' );
print Dumper( $ref2 );
# output: $VAR1 = undef;

my $ref3 = [];
testsub( $ref3, 'howdy' );
print Dumper( $ref3 );
# output: $VAR1 = [ 'howdy' ];

sub testsub
{
    my ( $ref, @args ) = @_;
    push( @$ref, @args );
}