use strict;
use warnings;

my $str = '<Data1>Data</Data1><Data2></Data2><Data3> < </Data3>';

# this code only works if same name tags are never nested
# in your XML-like samples.

$str =~ s/^\s+//;

my $sResult='';
while ($str =~ m{<(\w+)>((?:[^<]|<(?!/\1>))*)</\1>\s*}g) {
  my $tag = $1;
  my $innards = $2;

  $innards =~ s/</&lt;/;
  $innards =~ s/>/&gt;/;
  $sResult .= "<$tag>$innards</$tag>";
}
print STDERR "output: $sResult\n";