Your slice solution is buggy and uses scalar needlessly. 

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

[download]
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;

[download]

You need the conditional to avoid the warning.

Update: Added fix to OP's slice solution.

In reply to Re^3: Type of arg 1 to splice must be array (not keys) by ikegami
in thread Type of arg 1 to splice must be array (not keys) by jeanluca

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