The and is the conditional here. The equal sign = has a higher precendence than and and Perl guesses (it's a warning, not a syntax error) that you meant to have a comparison on its left side. The expression might also not do what you want since the result of and is not its second operand, but I guess you want "\${$1}" as replacement string.

The cleanest (IMO) way to get rid of that warning is to use do, as you did, but to replace the and in the do block with a semicolon:

do { $used_options{ $1 } = 1;  "\${$1}" }
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