Statements that sneakily alter the control flow are indeed bad style.

There are many alternative forms to choose from.

• You could set a flag and test that after the s///.
• Or a chain of m/\G.../g.
• You could embed the test condition in the regular expression: s/\$(\w++)(?(?{ !defined $defs{$^N} })(?!))/$defs{$1}/; Care must be taken that the whole text is matched: use anchors, possessive matching, etc.
• But if the %defs are not dynamic, the much simpler, and probably faster, approach is to construct the regex from your defs so that only valid tokens are accepted.
• Use something more capable for parsing, like Marpa.

Bottom line is, it sounds like you really need a lexer/scanner, not contorted regexes. Could you give more information pertaining to the problem at hand (the purpose of the wheel you're building)?

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