It's basically limited by the amount of memory on your machine.

On a Windows XP PC with 1 gigabyte of main memory and a Pentium IV, I can sort two million 24-bytes strings in just over half a minute, but trying to sort four million caused major thrashing (the whole machine slows to a crawl) followed by Perl dying with "out of memory."

Fifty thousand filenames shouldn't be a problem on reasonably recent machines.

perl -wle '@a=(1..2_000_000); @b=sort map {scalar gmtime $_} @a; print
+ "@b[0,1,2,1_999_997,1_999_998,1_999_999]"'
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Fri Jan  2 00:00:00 1970 Fri Jan  2 00:00:01 1970 Fri Jan  2 00:00:02 
+1970 Wed Jan 21 23:59:57 1970 Wed Jan 21 23:59:58 1970 Wed Jan 21 23:
+59:59 1970
34.42user 0.32system 0:35.76elapsed 97%CPU (0avgtext+0avgdata 29949952
+maxresident)k
0inputs+0outputs (122789major+0minor)pagefaults 0swaps
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