As Fletch suggested, a prototype will work in this situation:

#!/usr/bin/perl -w

use strict;

sub setmode ($\@) {
    my $val = shift;
    splice(@{$_[0]}, 1, 0, $val);
}

my @a = qw(one two three);

print join($/, @a), "$/__________$/";
setmode('add_test', @a);
print join($/, @a), $/;

__END__
result:

one
two
three
__________
one
add_test
two
three
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