It's not, you're using some form of interpolating quotes, \ escapes certain characters, invalid escape sequences return the original character. The \ get wiped out in the output.

You probably want to look at using Getopt::Std or Getopt::Long

--
perl -pew "s/\b;([mnst])/'$1/g"

In reply to Re: command line by belg4mit
in thread command line argument parsing (was: command line) by Sara

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