How to convert uppercase to lowercase with perl -pe?

When running Perl one-liners in a sandbox environment using perl - (which reads code from STDIN), you cannot use command-line flags like -pe or -e directly. The error happens because perl - expects Perl code on standard input, not command-line switches or scripts.

To convert uppercase letters to lowercase with transliteration (tr/A-Z/a-z/) inside a sandbox where you can’t pass command-line options, you need to write a small Perl script that mimics the behavior of perl -pe. The -p flag means “read each line from input, apply code to $_, then print $_ automatically.” You can duplicate this with a while(<>) loop and calling print explicitly.

Working Perl Example for Sandbox Execution

use strict;
use warnings;

# Simulate 'perl -pe "tr/A-Z/a-z/"' behavior

while (<>) {
    tr/A-Z/a-z/;    # transliterate uppercase to lowercase in $_
    print;          # print the modified line
}

__DATA__
Hello WORLD!
Perl IS Fun.
123 ABC xyz

This script reads lines from the DATA filehandle (literally the lines after __DATA__), converts uppercase characters to lowercase in place using tr/A-Z/a-z/, then prints each transformed line.

Key Perl Concepts

• $_ is the default variable for input line and string manipulation.
• while (<>) reads from @ARGV files or STDIN if none given, assigning each line to $_.
• tr/A-Z/a-z/ transliterates characters in place: here converting uppercase ASCII letters to lowercase.
• print outputs the modified line since there is no -p implicit printing.

Common Pitfalls

• Using perl -pe syntax with perl - (code on STDIN) causes syntax errors.
• tr/A-Z/a-z/ only handles ASCII letters; use lc for full Unicode lowercase.
• Carefully quote command-line one-liners to avoid shell or Perl parsing errors (not applicable here since code is inline).

Summary

If you want to run transliteration inside a sandbox without command-line flags, write a short Perl script with the common while (<>) loop, apply tr/A-Z/a-z/ to $_, and print. This approach works cleanly under perl - or embedded environments.