Also once I used '(?{})' with '(*FAIL)' simply for traversing all characters of the string.

    /.(?{ do_smth( $& ) })(*FAIL)/ for @lines;
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This part:

    /.(?{ do_smth( $& ) })(*FAIL)/
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is similar to:

    do_smth( $& ) while /./g;
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, but it can not(?) be combined with 'for' into one short expression like:

    do_smth( $& ) while /./g for @lines;  # syntax error
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Instead I should write:

    do{ do_smth( $& ) while /./g } for @lines;
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Another variant here was to concatenate all lines: my $concatenated_lines = join $some_concatenator, @lines. And then simply match. But imagine if the regex is more complex and can accidentally match a concatenator as well.
And another alternative here was to use eval within non-destructive substitution:

    s/./ do_smth( $& ) /ger for @lines;
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Note modifiers: e - for eval, and r - for non-destructiveness.
But imagine do_smth function returning long strings (e.g. ( ( ord $& ) % 2 ? $odds : $evens ) .= $&) and it could possibly(?) slow down process.