use strict;
use warnings;

while ( my $string = <DATA>) {
  $string =~ s/([A-Z]+)<(.+?)>/<\L$1\E>$2<\/\L$1\E>/g;
  print $string;
}

__DATA__
ABBR<a string>
ACRONYM<a string>
B<a string>
BIG<a string>
CITE<a string>
CODE<a string>
DFN<a string>
EM<a string>
I<a string>
KBD<a string>
SAMP<a string>
SMALL<a string>
SPAN<a string>
STRONG<a string>
SUB<a string>
SUP<a string>
TT<a string>
VAR<a string>
This is a B<string> which I want to I<parse> almost like EM<a POD>. CODE<Let's see if I can do it.>