Hm. See BUU for a simpler way to do it, but this seemed mildly interesting to me. This WTDI will produce a glob pattern that will match only the wanted files. (I'm currently on windows, so this will only ever give at most one filename, since the OS enforces case-insensitivity. This is, therefore, partaly tested.) Note that this won't work with real paths, only filenames, since the path seperator gets messed with. Makeiglob2 should get around this, but won't case-map things that aren't A-Za-z. A mix between them is possible that will work for both: change the m/^A-Za-z/ in makeiglob2 to m/<os-specifc path metachars>/. (os-specifc path metachars would be \ and : on win32, / on unix-likes, : on macos?) Most of the differences between these two, though, are in though-model. makeiglob is how I think. makeiglob2 is designed to be easy to follow for those newer to perl. Lists are happy things.

sub makeiglob {
 return join "", map "[\U$_\L$_]", split //, $_[0];
}
sub makeiglob2 {
 $out="";
 foreach (split //, $_[0]) {
  if (m/[^A-Za-z]/) {
   $out .= $_;
  } else {
   $out .= "[" . uc($_) . lc($_) . "]";
  }
 }
}

print glob(makeiglob(shift));
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File::Glob has a case insensitive mode, so assuming you have two files, abc.pl and abc.PL the following will display them both.

use File::Glob ':glob';
print "$_\n" for bsd_glob('*.pl', GLOB_NOCASE);
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However, NOCASE mode doesn't seem to be working properly if there are no wildcard characters in the glob:

use File::Glob ':glob';
print "$_\n" for bsd_glob('abc.pl', GLOB_NOCASE);
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That should display both filenames but it only returns abc.pl for me. (tested using 5.6.1 and 5.8.0 on RH7.3)

Is this a bug, or am I confused about how it should behave?

-Blake