You should be using

$today = &day($mon,$day,$year) ;
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instead of

$today = &day($day,$mon,$year) ;
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The following does the same, and it uses only core modules:

use POSIX       qw( strftime );
use Time::Local qw( timegm   );

print strftime("%a", gmtime(timegm(0, 0, 0, $day, $mon-1, $year)));
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or for current local time:

use POSIX       qw( strftime  );
use Time::Local qw( timelocal );

print strftime("%a", localtime);
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Use %A instead of %a if you are interested in the full name of the day instead of the three letter abbrieviation.