That's definitely a strange situation you've got there. The problem has something to do with the quantifier in the regex. When I replace /^([A-Z]+)$/ with /^([A-Z]*)$/ (i.e. change the '+' to a '*') I get... 
In if:   FOO
In else: 
In else: 
In if:   BAR
In else:

[download]
while changing the regex to /^([A-Z]{2,})$/ results in... 
In if:   FOO
In else: FOO
In else: 
In if:   BAR
In else: BA

[download]
While experimenting with {0,} {1,} {3,}, etc. it seems like in the non-matching case, $1 contains characters from the previous match and the number of them depends on the value for the minimum quantifier of the regex . I wouldn't try to depend on this behaviour if I were you:-)

In reply to quantifier optimization and dynamic scoping? by sleepingsquirrel
in thread What's happening to my $1? by Not_a_Number

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.