# global lookup hash
my %ESCAPES = (
	'&' => '&',
	'<' => '&lt;',
	'>' => '&gt;',
	'"' => '&quot;',
);

# the subroutine
sub xml_encode {
    my ($str) = @_;
    $str =~ s/([&<>"])/$ESCAPES{$1}/ge;
    return $str;
}

# and invoke it like
$data = xml_encode($data);