Perl sometimes gets "confused" about line numbers for errors that occur within conditional blocks. E.g., for something like

if ('s' =~ /s/) {  # error message may report this line number ...
    ...;
    ...;
    for (qw(q r s t u)) {
        s{ s }{ t }xms;  # ... for error on this line.
        }
    ...;
    ...;
    }  # ... or this line number ...
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Unfortunately, I can't reproduce this behavior with some nice example code ATM, so you'll have to take my word. Be that as it may, take a look inside the if-block for problematic code, substitutions in particular. Also, see "Modification of a read-only value attempted" in perldiag for some other suggestions.