(perl subs use call-by-value, which makes a copy)

I don't think this is true.

report_mem( 'program start' );
my $big = 'x' x 100_000_000;
report_mem( 'after making big string' );
take_big_arg( $big );

sub take_big_arg {
    printf "got %d characters\n", length $_[0];
    report_mem( 'passed to function' );
}

sub report_mem {
    my ( $msg ) = @_;
    printf "%d %s\n", my_mem(), $msg;
}

sub my_mem {
    my ($proc_info)
        = grep { $_->[2] == $$ } map { [ split ] } `ps l | tail -n +2`
+;
    return $proc_info->[6];
}
__END__
13124 program start
208444 after making big string
got 100000000 characters
208444 passed to function
[download]

The truth is Perl gives the sub aliases to the variables you pass. Common practice is to copy them after that (my ($copy) = @_), but that's something else.