# named alias:

sub func {    # usage:  func($string)
    for my $string ($_[0]) {
    $string =~ s/foo/bar/g;
    }
}

# direct @_ alias:

sub func {    # usage:  func($string)
    $_[0] =~ s/foo/bar/g;
}

# explicit reference:

sub func {    # usage:  func(\$string)
    my $stringref = $_[0];
    $$stringref =~ s/foo/bar/g;
}