Which version of List::Util? Upgrade List::Util? Try lc "$_"? What do you get with plain grep?

$ perl -MData::Dump -e "%f = qw/ A A B B C C/; dd( grep { lc $_ eq q/c
+/ } keys %f ); dd(\%f)"
"C"
{ A => "A", B => "B", C => "C" }
[download]

What do you get with non-hash?

$ perl -E"say grep { lc $_ eq q/c/ } qw/ A B C/"
C

$ perl -MList::Util=first -E"say first { lc $_ eq q/c/ } qw/ A B C/"
C
[download]