>perl -wMstrict -le
"my $prefix = qr{ (?i) \b tbgl_ }xms;
 my $word   = qr{ \w+ }xms;
 my $suffix = '_gl';
 ;;
 print '--- output ---';
 for my $pw (@ARGV) {
   (my $ws = $pw) =~ s{ $prefix ($word) }{$1$suffix}xmsg;
   print qq{'$pw' -> '$ws'};
   }
"
"tbgl_foo xxx %$# tbgl_bar()"
"TBGL_foo tBgL_bar"
"tbgl tbglfoo tbgl_"
"tbgl_%$# Xtbgl_foo"
--- output ---
'tbgl_foo xxx %$# tbgl_bar()' -> 'foo_gl xxx %$# bar_gl()'
'TBGL_foo tBgL_bar' -> 'foo_gl bar_gl'
'tbgl tbglfoo tbgl_' -> 'tbgl tbglfoo tbgl_'
'tbgl_%$# Xtbgl_foo' -> 'tbgl_%$# Xtbgl_foo'