The Perl 5 language interpreter.

# To parse and execute a Perl script:
perl script.pl

# To check syntax errors on a Perl script:
perl -c script.pl

# To parse and execute a perl statement:
perl -e perl_statement

# To run a Perl script in debug mode, using `perldebug`:
perl -d script.pl

# To loo[p] over all lines of a file, editing them [i]n-place using a find/replace [e]xpression:
perl -p -i -e 's/find/replace/g' filename

# To run a find/replace expression on a file, saving the original file with a given extension:
perl -p -i'.old' -e 's/find/replace/g' filename

# To run a multi-line find/replace expression on a file, and save the result in another file:
perl -p0e 's/foo\nbar/foobar/g' input_file > output_file