If you want to be mean... 
sub foo {
  my $condition = shift;
  my $some_var = generate_var() if $condition;
  # Use $some_var in an interesting way
}

# Elsewhere in code...
foo(0);
foo(1);
foo(0);

[download]
More reasonably, take strict out, and put a typo or two in a variable name. Use a hash, and have a typo in the hash key somewhere. Generate a report, and have the header and the fields not actually match up. Deliberately create a bug in one place, then put a comment in another suggesting that something like that bug would exist for another reason. Have a log with something like a free-form text field in it (an Apache error log would work) and ask why the number of records from processing it with a correct program don't match the number of lines in the log.

These are all (including the nasty at the start) based on real problems that I have encountered while debugging code.

In reply to Re: I need a "non-trivial" bug for my script! by tilly
in thread I need a "non-trivial" bug for my script! by Antigone

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.