That condition is so dense you missed that it devolves into an infinite loop of prints and warns. Fixed: 

while (1) {
   print "list of letters: ";
   defined( my $line = <> )
      or last;

   $line !~ /\s*quit|exit|q\s*$/i
      or last;

   # Do something with $line.
}

[download]

Update: Added missing my.

In reply to Re^3: $_ not set in while <> by ikegami
in thread $_ not set in while <> by pidloop

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