perl -e 'if (q{h@t} =~ /(\@)/ ) { print $1 }'
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See perlop for the q operator; look under "Regexp Quote-Like Operators", and a few scrolls down from that :)

PS: Your link to Re^3: Matching @ in string is broken because you did node=id instead of node_id - see What shortcuts can I use for linking to other information? for better, more efficient means of linking to information on Perlmonks.

... don't want to modify the string by escaping the @ and I want to run program as a one-liner, how do I do it if I can't escape the single quotes

Try: ...

 perl -e "print 'h@t'=~ /\@/g"
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 echo 'h@t' | perl -ne 'print /\@/g'
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... depending on the type of problem you are about to solve.

Regards

mwa