Is this magic enough for you?

#!/usr/bin/perl -w
# test.pl --
use strict;
use warnings;
use diagnostics;

my @a = qw( one two three );

print join( "\n", @a ), "\n__________\n";
setmode( 'add_test', \@a );
print join( "\n", @a );

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