c:\@Work\Perl\monks>perl -wMstrict -le
"my $user_input = 'ab+c';
 my $excluded = qr{ \A \Q$user_input\E _ABC_ .* [.] pdf \z }xms;
 ;;
 FILE:
 foreach my $file (qw(
   ab+c_ABC_something.pdf  ab+c_ABC_anything.pdf  ab+c_ABC_.pdf
   xab+c_ABC_.pdf  ab+c_ABC_.pdfx  xab+c_ABC_.pdfx
   abc_ABC_something.pdf  abc_ABC_anything.pdf  abc_ABC_.pdf
   abc  _.pdf  xyzabc_ABC_hipdip  foobar
   )) {
   next FILE if $file =~ $excluded;
   print qq{'$file' is not excluded};
   }
"
'xab+c_ABC_.pdf' is not excluded
'ab+c_ABC_.pdfx' is not excluded
'xab+c_ABC_.pdfx' is not excluded
'abc_ABC_something.pdf' is not excluded
'abc_ABC_anything.pdf' is not excluded
'abc_ABC_.pdf' is not excluded
'abc' is not excluded
'_.pdf' is not excluded
'xyzabc_ABC_hipdip' is not excluded
'foobar' is not excluded