triyng to lower case the to letters and replace the digits with yy

It would help if you'd include a sample of the desired output, rather than just a description and sample code. However, if I understand you correctly, this will do:

Outputs:

xyyy.TXT
twyy.TXT
kgyy.TXT
mmyy.TXT
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If you need the entire filename in lowercase, I might use another statement for clarity:

    $file =~ s/\d+/yy/;
    $file =  lc $file;
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