I suggest:

my @datafiles = glob("$basepath$ARGV[0]/$ARGV[1].[dD][aA][tT]");
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It takes care of the special cases and eliminates the grep.

Figuring out why it eliminates the grep is an exercise for the reader. :-) Hint: You might need to use man instead of perldoc. . .

-sauoq
"My two cents aren't worth a dime.";