AFAIR Perl caches the previous stringification of the regexp, and compares that against the stringification of the new pattern to determine whether it needs recompiling. The stringification of a compiled pattern always captures those outside flags that affect compile-time so as to make this (and interpolation) safe, that's why you see for example:

% perl -E 'say qr{foo}i'
(?^ui:foo)
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The /g flag is not listed in the docs because it is not supported by qr{}:

% perl -E 'say qr{foo}g'
Unknown regexp modifier "/g" at -e line 1, near "say "
Execution of -e aborted due to compilation errors.
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(I think that error message could probably be improved. Patches welcome. :)