Can I write useful Perl programs on the command line?

Yes. Read perlrun for more information. Some examples follow. (These assume standard Unix shell quoting rules.)

    # sum first and last fields
    perl -lane 'print $F[0] + $F[-1]' *

    # identify text files
    perl -le 'for(@ARGV) {print if -f && -T _}' *

    # remove (most) comments from C program
    perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c

    # make file a month younger than today, defeating reaper daemons
    perl -e '$X=24*60*60; utime(time(),time() + 30 * $X,@ARGV)' *

    # find first unused uid
    perl -le '$i++ while getpwuid($i); print $i'

    # display reasonable manpath
    echo $PATH | perl -nl -072 -e '
	s![^/+]*$!man!&&-d&&!$s{$_}++&&push@m,$_;END{print"@m"}'