Question: The one-liner should:

* solve a real-world problem
* not be extensively cryptic (should be easy to understand and reproduce)
* be worth the time it takes to write it (should not be too clever)

I'm looking for practical tips and tricks (complementary examples for `perldoc perlrun`).

Answer: 
All one-liners from the answers collected in one place: 

* `perl -pe's/([\d.]+)/localtime $1/e;' access.log`
* `ack $(ls t/lib/TestPM/|awk -F'.' '{print $1}'|xargs perl -e 'print join "|" => @ARGV') 
aggtests/ t -l`
* `perl -e'while(<*.avi>) { s/avi$/srt/; rename <*.srt>, $_ }'`
* `find . -name '*.whatever' | perl -lne unlink`
* `tail -F /var/log/squid/access.log | perl -ane 'BEGIN{$|++} $F[6] =~ m{\Qrad.live.com/ADSAdClient31.dll}
&& printf "%02d:%02d:%02d %15s %9d\n", sub{reverse @_[0..2]}->(localtime $F[0]), @F[2,4]'`
* `export PATH=$(perl -F: -ane'print join q/:/, grep { !$c{$_}++ } @F'<<<$PATH)`
* `alias e2d="perl -le \"print scalar(localtime($ARGV[0]));\""`
* `perl -ple '$_=eval'`
* `perl -00 -ne 'print sort split /^/'`
* `perl -pe'1while+s/\t/" "x(8-pos()%8)/e'`
* `tail -f log | perl -ne '$s=time() unless $s; $n=time(); $d=$n-$s; if ($d>=2) { print qq
($. lines in last $d secs, rate ),$./$d,qq(\n); $. =0; $s=$n; }'`
* `perl -MFile::Spec -e 'print join(qq(\n),File::Spec->path).qq(\n)'`

See corresponding answers for their descriptions.
I often need to see a readable version of the PATH while shell scripting. The following one-liners print every path entry on its own line.

Over time this one-liner has evolved through several phases:

Unix (version 1):

```
perl -e 'print join("\n",split(":",$ENV{"PATH"}))."\n"'

```

Windows (version 2):

```
perl -e "print join(qq(\n),split(';',$ENV{'PATH'})).qq(\n)"

```

Both Unix/Windows (using q/qq tip from @j-f-sebastian) (version 3):

```
perl -MFile::Spec -e 'print join(qq(\n), File::Spec->path).qq(\n)' # Unix
perl -MFile::Spec -e "print join(qq(\n), File::Spec->path).qq(\n)" # Windows

```