$ perl -Mre=debug -e'qr/foo|bar|baz|boo/'
Compiling REx "foo|bar|baz|boo"
Final program:
   1: TRIEC-EXACT[bf] (13)
      <foo>
      <bar>
      <baz>
      <boo>
  13: END (0)
stclass AHOCORASICKC-EXACT[bf] minlen 3
Freeing REx: "foo|bar|baz|boo"

$ perl -Mre=debug -e'qr/foo|bar|baz|boo/i'
Compiling REx "foo|bar|baz|boo"
Final program:
   1: BRANCH (4)
   2:   EXACTF <foo> (13)
   4: BRANCH (7)
   5:   EXACTF <bar> (13)
   7: BRANCH (10)
   8:   EXACTF <baz> (13)
  10: BRANCH (FAIL)
  11:   EXACTF <boo> (13)
  13: END (0)
minlen 3
Freeing REx: "foo|bar|baz|boo"