(updated) In your "if" construct, the "!=" relationship is coercing both sides to numeric. The effect on the if statement is that it is equivalent to /^(\d+)/ and ( $1 ) and $exmno=0; which is probably not what you wanted. If the line were stored in a declared variable, what you would want instead would be
if ( $variable !~ /^<exm>/ ) { $exmno = 0 );

[download]
but because the variable to match is $_, this is the default variable for the match operator and it is therefore sufficient to write:
if ( !/^<exm>/ ) { $exmno = 0 };

[download]
or even shorter is 
/^<exm>/ or $exmno = 0;

[download]
Two further points are that 1) this test is anyway redundant because s// returns false for non-match and can be used directly to drive the reset to zero and 2) if the counter is zero until it encounters an exm line, and if the first exm line in a group is number 1, then the counter needs to be pre- rather than post-incremented, e.g.:
my $exmno = 0;
while(<>) {
    s/^<exm>/'<exm num ='.++$exmno.'>'/ or $exmno = 0;
    print $_;
}

[download]

-M

Free your mind

In reply to Re: Restarting counters in text by Moron
in thread Restarting counters in text by rsriram

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.