Have a look at ref. It'll tell you what kind of reference the argument is, or return an empty string if it's a scalar. For example:

my $hashref  = {};
my $arrayref = [];
my $coderef  = sub {};
my $scalar   = '';

print "[", ref, "]\n"
    for $hashref, $arrayref, $coderef, $scalar;
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Outputs

[HASH]
[ARRAY]
[CODE]
[]
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