Your slice solution is buggy and uses scalar needlessly.

my $keys = keys(%my) < 100 ? keys(%mh) : 100;
my @k = ( keys %mh )[0..$keys-1];
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or

my $keys = min 100, keys(%mh);
my @k = ( keys %mh )[0..$keys-1];
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(List::Util provides min)

A splice solution:

my @k = keys %mh;
splice( @k, 100 ) if @k > 100;
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You need the conditional to avoid the warning.

Update: Added fix to OP's slice solution.