This should be pretty definitive...

use strict;
use warnings;

sub tst {
    warn "entering tst()\n";
    my $arg = shift;  # line 6
    warn "leaving tst()\n\n";
}

my %h=();
tst $h{''};
tst $h{undef};  # line 12
my $bla='';
tst $h{$bla};  # line 14
$bla=undef;
tst $h{$bla};  # line 16
undef $bla;
tst $h{$bla};  # line 19
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And here's the results:

entering tst()
leaving tst()

entering tst()
leaving tst()

entering tst()
leaving tst()

Use of uninitialized value $bla in hash element at perlmonks-1063097.p
+l line 16.
entering tst()
Use of uninitialized value in scalar assignment at perlmonks-1063097.p
+l line 6.
leaving tst()

Use of uninitialized value $bla in hash element at perlmonks-1063097.p
+l line 18.
entering tst()
Use of uninitialized value in scalar assignment at perlmonks-1063097.p
+l line 6.
leaving tst()
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Turning off 'strict' has no effect, while turning off 'warnings' warnings gets rid of the messages.

I'm actually surprised by a couple of aspects of this. First, I expected $bla=undef; tst $h{$bla} to behave the same as tst $h{undef}; it didn't. Instead, $bla=undef seems to have the same effect as undef $bla. Second, it seems that lines 16 and 19 are generating a value other than a simple undef to pass into the subroutine, otherwise the invocation in line 12 would have also generated an error in line 6.