... 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"
[download]

or even

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

Regards

mwa