For example, the following code will print bar! if the command line switch -foo is passed to the script:

#!/usr/bin/perl -s

print "bar!\n" if ($foo);
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This behaviour is documented in perlrun - For a truly evil use of this behaviour (and Perl in general) have a look at theDamian's selfGOL obfuscation :-)

perl -e 'print+unpack("N",pack("B32","00000000000000000000000111011111")),"\n"'

How does the use of $foo as a switch jive with the recomendation to use strict; for all programs? Can a switch be localized? Are switches only possible in programs without use strict;? How would switches be declared (?) for use in my program if I do want to use strict;?

I think I've got some research to do when I get home.

How does the use of $foo as a switch jive with the recomendation to use strict; for all programs?

In short, it doesn't - The -s argument is meant for quick-and-nasty argument handling, the usage of these variable names causing explicit package name errors when employed with strict.

For example:

rob@budapest:/home/rob# cat test.perl
#!/usr/bin/perl -s

use strict;

print "bar!\n" if ($foo);

rob@budapest:/home/rob# ./test.perl -foo
Variable "$foo" is not imported at ./test.perl line 5.
Global symbol "$foo" requires explicit package name at ./test.perl lin
+e 5.
Execution of ./test.perl aborted due to compilation errors.
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If however, strict is turned off within the scope where these variables are employed, everything is happy once more. Eg.

rob@budapest:/home/rob# cat test.perl
#!/usr/bin/perl -s

use strict;

{
    no strict;
    print "bar!\n" if ($foo);
}

rob@budapest:/home/rob# ./test.perl -foo
bar!
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perl -e 'print+unpack("N",pack("B32","00000000000000000000000111100000")),"\n"'

I would recommend using one of the GetOpt modules for anything other than a throw-away script.

--
I'm not belgian but I play one on TV.

here's how I used to do it before I learned getopt:

it uses grep, puts your arguments in a hash, and you can easily set defaults, and it automatically makes your "usage" string, and it automatically rejects invalid arguments. arguments are in "option=value" style to avoid ambiguity

#!/usr/bin/perl -w

# set defaults here
%args = ( width => 750,
          height => 600,
          invert_threshold_factor => 0.25,
          invert => -1,
          pscanhome => "/home/lab/pscan",
          color_a => "darkcyan",
          color_b => "darkred");

$options = '[' . (join '=?] [', sort keys %args) . '=?]';
$usage = "usage: $0 $options picname\n";
$name = pop || die $usage;

foreach (@ARGV) {
   my ($a, $b) = split '=';
   die $usage unless grep {$a eq $_} keys %args;
   $args{$a} = $b;
}
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