if ( $string =~ m/abc(?:def|xyz)/ ) { 
  print "ok!" 
}
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You could do the same with:

if ( $string =~ m/abc(def|xyz)/ ) { 
  # I didn't use ?:
  print "ok!" 
}
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but in this case the regexp would spit out in $1 'def' or 'xyz', which probably you don't want. Look at the Regexp Documentation for further details.