If you aren't using your STDERR for anything else, just redirect it to the file and the warnings will show up there. You can do that in your shell script with foo.pl 2>> errlog.txt, or in Perl with:

open STDERR, ">errlog.txt"
  or die "Whoa! I can't even log my errors: $!";
[download]

Otherwise, you'll want the __WARN__ pseudo-signal:

open ERRLOG, ">errlog.txt" or die;
$SIG{__WARN__} = sub {
  my ($msg) = @_;
  $msg ||= q{Warning: something's wrong};

  # Add the file and line number to $msg here
  # using the return values from caller().

  print ERRLOG, $msg;
};
[download]

See also warn and caller in perlfunc and %SIG in perlvar.

---

---

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! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

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:
	&		&
	<		&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.