Poking around in the source, I find that the error:

  Can't coerce UNKNOWN to string in aelem ... line 68 ...
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is generated when Perl_sv_pvn_force_flags is presented with a value which is an Array, Hash, Code or IO or some completely bogus type: (SvTYPE(sv) > SVt_PVLV && SvTYPE(sv) != SVt_PVFM).

The lines around the reported error location are:

  67:  if (defined($text->[0])) {
  68:    my @text = collapse(@$text);
  69:    print $fh_ERR "$url|", join( ",", @text ), "\n";
  70:  }
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The aelem suggests that the error is in an array element look up... however, things are all a bit mysterious:

• there an array look up on line 67, but not on line 68.

• I'm not convinced I can think of a case where aelem would be doing a force to string (PV).

• I cannot see anywhere where defined would set about forcing to PV

• the join on line 69 looks like the best candidate for forcing to PV -- but I don't see a good reason for the error being reported on the wrong line or in the wrong operation...

• the reported UNKNOWN type is a BIG WORRY... because it indicates that whatever the SV is that is being forced to string, it's not of any type known to Perl -- which suggests something has gang awry in an omigod-could-this-be-a-bug-in-Perl sort of a way.

It could be time to get out the debugger and place a breakpoint on the:

  if (SvTYPE(sv) > SVt_PVLV && SvTYPE(sv) != SVt_PVFM)
    Perl_croak(aTHX_ "Can't coerce %s to string in %s",
                 sv_reftype(sv,0),
                 OP_NAME(PL_op));
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in Perl_sv_pvn_force_flags and having a poke around in the entrails.