"Quick and Dirty" refers to small, hastily written scripts
that aren't particularly robust or optimized. They often accomplish a specific, narrowly-defined goal, and can't be easily reused for similar tasks. In this context a program is "dirty" if it doesn't make the job of writing the next one any easier. Perl excels in this domain because it doesn't impose many restraints on the programmer. Other languages adopt various formalisms that are good practices for bigger projects, but overkill for a 10 line program.
Perl's native abilities are also often a great match with
the needs of the typical one-off script.
Of course, perl allows you to enable stricter coding guidelines with various pragmas such as 'use strict', and 'use warnings' (and actually 'use Module' in general makes for less-dirty code...). Hence the mantra "Perl makes the easy things easy and the hard things possible."