Your code does not compile: strict ends in a "t".

If you are using the /e option of the substitution, make sure the string is a valid Perl expression:

$file =~ s/(\w{2})(\d+)/$1 . lc($1) . $2 . lc("yy")/e;
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Update: If you do not want to use the numbers, do not capture (and output) them:

$file =~ s/(\w{2})\d+/$1 . lc($1) . "yy"/e;
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Note that you can use the \L special character instead of the lc function, which means you do not need the /e option at all:

  $file =~ s/(\w{2})\d+/\L$1yy/;
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