use YAPE::Regex::Explain;

print YAPE::Regex::Explain->new(qr/(^USA|BRA|CAN)?(.*?)(XY)?$/)->expla
+in();
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The regular expression:

(?-imsx:(^USA|BRA|CAN)?(.*?)(XY)?$)

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):
----------------------------------------------------------------------
  (                        group and capture to \1 (optional
                           (matching the most amount possible)):
----------------------------------------------------------------------
    ^                        the beginning of the string
----------------------------------------------------------------------
    USA                      'USA'
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    BRA                      'BRA'
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    CAN                      'CAN'
----------------------------------------------------------------------
  )?                       end of \1 (NOTE: because you are using a
                           quantifier on this capture, only the LAST
                           repetition of the captured pattern will be
                           stored in \1)
----------------------------------------------------------------------
  (                        group and capture to \2:
----------------------------------------------------------------------
    .*?                      any character except \n (0 or more times
                             (matching the least amount possible))
----------------------------------------------------------------------
  )                        end of \2
----------------------------------------------------------------------
  (                        group and capture to \3 (optional
                           (matching the most amount possible)):
----------------------------------------------------------------------
    XY                       'XY'
----------------------------------------------------------------------
  )?                       end of \3 (NOTE: because you are using a
                           quantifier on this capture, only the LAST
                           repetition of the captured pattern will be
                           stored in \3)
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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This is then fed into the Conditional Operator (? :), selecting either the second group ((.*?)) if it matched or the whole string if it didn't. Finally, this resultant string is used as a symbolic reference. Good luck. I suspect that the author intended (^USA|BRA|CAN) to actually be ^(USA|BRA|CAN)

You already got a nice answer from kennethk. I just wanted to add that you may want to move the caret left one position though: Currently USA must be at the start of the string, but BRA and CAN can be anywhere....

...roboticus

When your only tool is a hammer, all problems look like your thumb.

Would this be OK or the right thing is to move the caret left outside?
Could the first way work?

${ ($acc =~ /(^USA|^BRA|^CAN)?(.*?)(IN)?$/i) ? $2 : $acc }
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or

${ ($acc =~ /^(USA|BRA|CAN)?(.*?)(IN)?$/i) ? $2 : $acc }
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The results in the two cases are identical since ^ is a zero-width match. I personally think the second option is better because it is clearer and has fewer characters, hence is less sensitive to typos, but that is wholly subjective.

This statement simply removes one of "USA", "BRA" or "CAN" from the start of the string and "XY" from the end of string, if found.