Set up CPAN

perl -MCPAN -e shell

Find modules currently installed

find `perl -e 'print "@INC"'` -name '*.pm' -print

Taint mode

For untrusted scripts, add -T flag to perl at the start of the script:
#!/usr/local/bin/perl -T

One liner to edit files in place

This command edits a group of files in place. In particular, it does a global search/replace. The -p switch tells perl to iterate over filename arguments, -i tells perl to edit files in place, and -e indicates that a one line program follows.
perl -p -i -e 's/248/949/g' *.txt

One liner to edit files in place and backup each one to filename.old

perl -p -i.old -e 's/248/949/g' *.txt