sub transform {
   my ($ref) = @_;
   $$ref = 'tripped';
}

$text = 'The brown cow jumped over the moon';

if ( $text =~ m/cow\s+(jumped)\s+over/ ) {
    my $ref = \substr($text, $-[1], $+[1] - $-[1]);
    transform($ref);
}

print("$text\n");  # The brown cow tripped over the moon
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Update: Expanded the example to be runnable.