use strict;

sub exists_glob {
  my $tbl  = \%main::;
  $tbl = $tbl->{"$_\::"}
    for split '::', scalar caller;
  return exists $tbl->{$_[0]} && 'GLOB' eq ref \$tbl->{$_};
} 

$main::{foo} = "I'm a string";
*bar         = \"I'm in a glob";

printf "%s is%s a glob\n", $_, (exists_glob($_) ? '' : "n't")
  for qw/ foo bar /;

__output__

foo isn't a glob
bar is a glob