I can't reproduce your problem (on the W7 machine available right now). 

C:\>perl -p -w -e "s/(\w+)/\u\L$1/;"  # quoting changed from OP to obl
+ige M$
foo
Foo
....

[download]

Beyond that, and even though none of this is really on point:

  1. It's not, as your title has it, "the same script." The first version provides an explicit means, <>, of getting a "word" into the "script" while the second uses one version of a 'wrap in a loop' feature ,-p. (BTW, you can concatenate the options as -pwe ... or in 5.14 -- perhaps earlier -- as -pwE to enable features like say. If you do so, the -w becomes unnecessary, since recent versions of Perl -- when executed with features enabled -- automatically invoke use strict; use warnings;.)
  2. The \L in the substitution can be omitted because you use \u which uppercases the first letter only. An upper case \U would say 'uppercase the rest of the word' (and gets Perl to squawk if used together with <c>\L<c>
Updated: typo fixed

In reply to Re: same script acts differently when run as a file vs one-liner by ww
in thread same script acts differently when run as a file vs one-liner by mavili

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.