sub convert {
    ### This subroutine accepts a single parameter (a scalar which it treats as a string)
    ### and sequentially substitutes out "invalid" characters with their "valid" counterparts
    ### based on a fixed list of characters to replace.
    my $input_stream = shift;
    return undef unless $input_stream;
    my %char_swap_hash = (
        1 => {OLD => '&[^(amp;)|(lt;)|(gt;)]', NEW => '&'}, # don't match legit '&' codes
        2 => {OLD => '<', NEW => '&lt;'},
        3 => {OLD => '>', NEW => '&gt;'}
        );
    $input_stream =~ s/$char_swap_hash{$_}{OLD}/$char_swap_hash{$_}{NEW}/g for keys %char_swap_hash;
    return $input_stream;
}