if there are say 50,000 filenames then will that array takes those names??

Easily. 50,000 entries take about 50,000 * 4 = 200Kb for the pointers + the memory for the filenames, say 200 * 50,000 = 1 Mb plus some overhead. About 2 Mb.

Perl arrays are implemented as arrays of C pointers so you'll probably run out of memory for the data in it before you reach the maximum length. Theoretically a perl array has a 2**31 = 2147483648 element limit. On a 32 bit perl / os you'll run out of memory for the array itself somewhere before half that amount.

update: look up "AV" in http://gisle.aas.no/perl/illguts/ if you're really interested.