You can get an explanation of any regular expression using YAPE::Regex::Explain. In this case, it outputs:

The regular expression:

(?-imsx:^.{!}(.{5}))

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  .                        any character except \n
----------------------------------------------------------------------
  {!}                      '{!}'
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    .{5}                     any character except \n (5 times)
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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The issue is that the $! variable is interpolated to an empty string but ! operator doesn't get interpolated as such in the regular expression. Essentially, your modification is actually looking for the string literal {!}.