You just go ahead and try it from the command line (make sure that warnings are enabled):

% perl -wle 'my %foo; if ( %foo ) { print "OK" } else { print "KO" }'
KO
% perl -wle 'my %foo = (1,2); if ( %foo ) { print "OK" } else { print 
+"KO" }'
OK
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But you have to be careful, because sometimes perl will conjure keys from thin air. For example:

% perl -wl
print( %foo ? "OK" : "KO" );
my @bar = grep $_, @foo{ 1, 2, 3 };
print( %foo ? "OK" : "KO" );
^D
KO
OK
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Notice how the second test succeeded, even though we never initialized %foo. For this reason, maybe something like this (also proposed by brian_d_foy) would be better:

if ( grep defined, values %foo ) {
  # etc.
}
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This test will succeed only if %foo contains at least one defined value.