I have no solution to the eval problem, but FWIW, MO=Deparse,-p clarifies the issue: 

% perl -MO=Deparse,-p -le '$patt = "a\Ubc"; print qr/$patt/'
BEGIN { $/ = "\n"; $\ = "\n"; }
($patt = 'aBC');
print(qr/$patt/);
-e syntax OK

[download]
Likewise for \Q...\E: 
% perl -MO=Deparse,-p -le '$patt = "a\Q[bc]\E"; print qr/$patt/'
BEGIN { $/ = "\n"; $\ = "\n"; }
($patt = 'a\\[bc\\]');
print(qr/$patt/);
-e syntax OK

[download]
In other words, by the time that qr sees it, perl has already transformed the original string, which explains both the problem and the eval workaround.

Note that this behavior occurs only if the RHS of the first assignment is double-quoted; with single-quotes you end up with the same problem as if the pattern had been passed in in @ARGV: 

% perl -le '$patt = q(a\Ubc); print qr/$patt/'
(?-xism:a\Ubc)

[download]

Update: Deleted a bit of stray text that had snuck into one of the original code snippets; added the output for the \Q...\E case.

the lowliest monk

In reply to Re: qr/string/ is not the same as qr/$var/ ? by tlm
in thread qr/string/ is not the same as qr/$var/ ? by grinder

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.