Thanks for the replies.

Are there modules or debugging techniques that can help track down such a logical $1 error? Consider the code below. 

print chg("ctag\n", qr/t(g)/);   # print "ctag"
"g" =~ /(c)/;
print chg("ctag\n", qr/t(g)/);   # print "ctag"

print chg("ctag\n", qr/t(g)/);   # print "ctag"
"g" =~ /(g)/;
print chg("ctag\n", qr/t(g)/);   # print "ctga"

sub chg {
    my ($str, $pat) = @_;
    $str =~ /$pat/;
    eval "\$str =~ tr/a$1/$1a/" if $1;
    return $str;
}

[download]

The code will work most of the time except when something like "g" =~ /(g)/ occurs. If I'm using someone else's code or even my own (I can't force someone or myself to write bug-free code), since the logical error occurs only once a while and even if it happens it's not necessarily noticeable, when such an error does occur, $1 is probably not the first thing coming to my mind when trying to debug it, especially when there're no warning messages pointing me to it.


__________________
Are these similar issues whenever using such global variables as $_?

Update: Maybe I should rephrase my question as not just how to avoid this $1 mistake in the first place but how to pinpoint one afterwards should an error already be happening.

In reply to Re: $1 trap by chunlou
in thread $1 trap by chunlou

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