These are probably stupid-simple-obvious, but I find them useful enough that they made it into little scripts in ~/bin.
perl -ne'print unless $lines{$_}++'
Only print the first occurance of a line. This one came about recently when I wanted to find out which of my ripped albums were 128kbps mp3s (so I could re-rip them!):
mp3find ~/cds -bitrate 128 -printf '%b' | unique
perl -pe'chomp;$_=quotemeta($_)."\n"'
This one comes in handy when using xargs to pipe a bunch of mp3 filenames with spaces in them to something like madplay or BMP.
perl -pe's/(^|\s)#.*//;'
Dumb comment stripper. Again useful for piping M3U lists to madplay:
nocomment ilyaimy.m3u | qm | xargs madplay
