#! perl -sw
use strict;

while( <DATA> ) {
    s[^(<([^>]+?)>)(.*)(</\2>)]{
        (my $x = $3) =~ tr[<>][[]];
        "$1$x$4";
    }e;
    print;
}


__DATA__
<sometag> my data here </sometag>
<anothertag> further text </anothertag>
<furthertag> not good as contains <this> in angle brackets </furthertag>
<byetag> another possibility is <more> <than> one <angle pairs> in here </byetag>
<a tag with spaces> and content containing a false </a tag with spaces> and a real </a tag with spaces>