You might be looking for ref()

> perl -e '$u='hallo'; print ref($u),"\n"; '

> perl -e '$u=\'hallo'; print ref($u),"\n"; '
SCALAR
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UPDATE: Misread your question. You are looking for the symbol table hashes

> perl -e ' if (exists $main::{"u"}) { print "yes\n"; } '

> perl -e '$u='hallo'; if (exists $main::{"u"}) { print "yes\n"; } '
yes
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