The problem with your original version is that the use constant executes at compile time, but the regular expression executes at run time - i.e. after everything has been compiled - so it's setting the $CONF_PATH variable too late. To see the difference between compile time and run time, run this:

print "World\n";
BEGIN { print "Hello " };

[download]

And the use keyword (plus its friend no) runs conceptually in a BEGIN block.

A do { ... } block provides a neat solution:

package mylib;
use constant {
    MODULE_PATH => __FILE__,
    CONF_PATH   => do { $_ = __FILE__; s/pm$/conf/i; $_ },
};
print CONF_PATH;

[download]

If your perl is fairly recent, you can do away with do and use /r.

package mylib;
use constant {
    MODULE_PATH => __FILE__,
    CONF_PATH   => (__FILE__ =~ s/pm$/conf/ir),
};
print CONF_PATH;

[download]
perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

In reply to Re: BEGIN block and failure to initialize constants by tobyink
in thread BEGIN block and failure to initialize constants by temporal

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.