The simplest way is a one-liner: 

  perl -i.bak -pe 's/hello/goodbye/gi' file1 file2 ...

[download]
You are reading the file and modifying an in-memory string, but not attempting to write that string back out. I would do this: 
  use Fcntl qw(:seek);

  open (FOO, "+<$foo") or die "Can't open $foo: $!";
  my $slurp = join '', <FOO>;
  $slurp =~ s/hello/goodbye/sgi;
  seek FOO, 0, SEEK_SET;
  print FOO $slurp;
  truncate FOO, tell FOO;
  close FOO;

[download]
Update: Added the truncate call for cases where the modified file contents are shorter than the original. Oops.

In reply to Re: Editing Contents of a File by pbeckingham
in thread Editing Contents of a File by bivouac

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.