If your program is not being run on the command line you could use code similar to this. 
my $filename = 'foo.txt';

open IN, "< $filename" or die "Can't open $filename: $!\n";
open OUT, "> $filename.bak" or die "Can't write to $filename: $!\n";
while (<IN>) {
  s/foo/blah/g;
  print OUT $_;
}
close IN; close OUT;
rename "$filename.bak", $filename;

[download]
However this code is only the barest code that will work, and should probably not be used without file locks and other sanity checks.

In reply to Re: general perl question by repson
in thread In-place editing of files (was: general perl question) by cghost23

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.