I would implement the algorithm in nearly the same way, but I think I would implement it as a filter-like command which you can easily call in pipe sequences. So read-in with the magic <> (this allows for STDIN or infiles) and print to STDOUT (which can be easily redirected into outfile from the command line). This has the additional advantage of shortening the program a bit:

#!/usr/bin/perl -w
use strict;
 
my $uclc = shift or Usage();

# fixed small bug
Usage() unless ($uclc eq 'lc' or $uclc eq 'uc');

# to speed up the loop (no string compare on every line)
my $lc = $uclc eq 'lc';
 
while (<>) {
  print $lc ? lc : uc;
}
 
sub Usage {
   die << EO_USE;
Usage: uclc.pl (lc|uc) [infile]
reads from STDIN if no infile given
EO_USE
}
[download]

Update: Oops, thanks tilly, fixed: while instead of foreach.

-- Hofmator

Change the foreach to a while.

The difference is that foreach puts the file read into list context so the entire file has to be held in memory. The while prints as it reads and so will work much better on large files.

Otherwise I like your changes.

I'm not familiar with the eq 'lc' ? lc() : uc(); syntax from print OUT $uclc eq 'lc' ? lc() : uc();.   Would you be so kind as to explain how that works?

Updated: 2001-07-29 07:40 CDT

Here's what I ended up using.   Thanks again, and ++ to all who offered suggestions in nodes and CB!
    cheers,
    Don
    striving toward Perl Adept
    (it's pronounced "why-bick")

#!/usr/bin/perl -w

use strict;

my $uclc = shift or Usage();
Usage() unless ($uclc eq 'uc' or 'lc');
while(<>) { print $uclc eq 'uc' ?  uc() : lc(); }

sub Usage {
   print "\n Usage: uclc.pl (uc|lc) < infile > outfile\n\n";
   exit;
   }

=head1 NAME

 uclc.pl

=head1 SYNOPSIS

 uclc.pl (uc|lc) < infile > outfile"

 Convert alpha characters in text file to uppercase (or lowercase).

=head1 UPDATED

 2001-07-26 16:30 CDT
   Simplify code for (in|out)put of STD(IN|OUT).
   Remove unnecessary "or Usage()" from first two input lines.
   Replace if/else with ternary "?:".
     my $munged;
     if ($uclc eq 'uc') { $munged = uc(); }
     else {$munged = lc(); }
     print OUT $munged;
   While instead of for so file read line-by-line not slurp entire fil
+e.
   Post efficiency SoPW to PerlMonks
 2001-07-25
   Initial working code.

=head1 TODOS

 None that I know of.

=head1 TESTED

 ActivePerl 5.61 on Win2kPro

=head1 AUTHOR

 ybiC

=head1 CREDITS

 Thanks to synapse0, OeufMayo, crazyinsomniac, Masem,
 MZSanford, virtualsue, Hoffmater, tilly, clemburg,
 and ichimunki for suggestions.
 And to davorg for "Data Munging with Perl".
 Oh yeah, and to some guy named vroom.

=cut
[download]

from perlop:

 Conditional Operator

   Ternary "?:" is the conditional operator, just as in C. It works mu
+ch
   like an if-then-else. If the argument before the ? is true, the arg
+ument
   before the : is returned, otherwise the argument after the : is
   returned. For example:

       printf "I have %d dog%s.\n", $n,
               ($n == 1) ? '' : "s";
[download]

for the gory details read on there ...

-- Hofmator

see perlman:perlop and look for the section on Conditional Operator. It's basically an in-place if-then type of construct that evaluates the EXPR $uclc eq 'lc' and returns lc() if the EXPR is true or uc() if the EXPR is false.