cat p.pl; echo ; perl -Dr p.pl

printf "matched: %s\n"
  , ( 'x' =~ undef() ? 'yes' : 'no' )
  ;

Omitting $` $& $' support.

EXECUTING...

Compiling REx ""
Final program:
   1: NOTHING (2)
   2: END (0)
minlen 0
Matching REx "" against "x"
   0 <> <x>                  |  1:NOTHING(2)
   0 <> <x>                  |  2:END(0)
Match successful!
matched: yes
Freeing REx: ""