reftype(\$s) eq 'GLOB' will be true if and only if $s contains a glob.

$ perl -MScalar::Util=reftype -e'
   my $s = *STDOUT;
   CORE::say reftype(\$s) eq "GLOB";
'
1
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reftype($s) eq 'GLOB' will be true if and only if $s contains a reference to a glob.

$ perl -MScalar::Util=reftype -e'
   my $s = \*STDOUT;
   CORE::say reftype($s) eq "GLOB";
'
1
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This is what open(my $fh, ...) places in $fh.

$ perl -MScalar::Util=reftype -e'
   open(my $s, "<&", *STDOUT) or die $!;
   CORE::say reftype($s) eq "GLOB";
'
1
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