I find it surprising that adding the modifier back later has no effect, and triggers no warnings.

It does have an effect. The effect is on the pattern you add it to though, not on the compiled pattern. The following illustrates what I mean: 

$ perl -le '$r = qr/qux./; $s = "qux\n"; print "matches" if $s =~ /$r/
+'
$ perl -le '$r = qr/qux./; $s = "quxxx"; print "matches" if $s =~ /$r.
+/'
matches
$ perl -le '$r = qr/qux./; $s = "quxx\n"; print "matches" if $s =~ /$r
+./'
$ perl -le '$r = qr/qux./; $s = "quxx\n"; print "matches" if $s =~ /$r
+./s'
matches

[download]

-sauoq
"My two cents aren't worth a dime.";

In reply to Re^3: Pattern matching with qr// and modifiers by sauoq
in thread Pattern matching with qr// and modifiers by Athanasius

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.