$string =~ m/^(frm)(\w+?)(txt)?$/;
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There's a problem here in that if  'txt' may not be present (as  (txt)? implies) and an overall match is still desired,  $3 will be undefined because the third capture group need not match at all. A different approach will yield an empty string instead.

>perl -wMstrict -le
"for (@ARGV) {
   m/^(frm)(\w+?)(txt)?$/;
   print qq{'$_' ($1) ($2) ($3)};
   }
 print '-----------';
 for (@ARGV) {
   m/^(frm)(\w+?)(txt|)$/;
   print qq{'$_' ($1) ($2) ($3)};
   }
"  frmNametxt  frmNameFoo
'frmNametxt' (frm) (Name) (txt)
Use of uninitialized value $3 in concatenation (.) or string at ...
'frmNameFoo' (frm) (NameFoo) ()
-----------
'frmNametxt' (frm) (Name) (txt)
'frmNameFoo' (frm) (NameFoo) ()
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