What are common pitfalls or gotchas with qr// precompiled regex?

When using the qr// operator in Perl for precompiled regex, there are several common pitfalls and gotchas to be aware of:

• Variable Interpolation: Unlike regular regex, if you use variable interpolation inside the pattern, you might not get the expected results. Perl does not interpolate variables within the qr// unless you use the /e modifier or create a double-quoted string.
• Modifiers: Be cautious about the modifiers you apply with qr// (like /i for case insensitive). They are compiled into the regex object but may not behave as expected if you're changing them dynamically.
• Scope: Remember that the compiled regex can be scoped, which means if defined in a block, it may not be available outside of that block, leading to 'Undefined subroutine' errors.
• Performance Considerations: While precompiled regex can speed up matched operations, they could inadvertently decrease performance if misused, such as in cases of large repetitive matching operations.
• Unicode & Encodings: If working with Unicode, ensure you have the correct encoding in mind when compiling your regex, or you might not match characters as expected.

Here's an example of using qr// for a precompiled regex in Perl: