I’m not familiar with the Crypt::OpenPGP module, but from the sourcecode:

sub encrypt {
    ...
    if ($param{SignKeyID}) {
        $ptdata = $pgp->sign(
        ...
        KeyID => $param{SignKeyID},
        ...
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and from the documentation for $pgp->sign( %args ):

%args can contain:

• ...
• KeyID
  The ID of the secret key that should be used to sign the message. The value of the key ID should be specified as a 16-digit hexadecimal number.
  This argument is mandatory.

So SignKeyID is required to be a 16-digit hexadecimal number. I hope this is helpful.