A more concise alternative is: 

c:\@Work\Perl\monks>perl -wMstrict -le
"use warnings;
 use strict;
 ;;
 use Test::More 'no_plan';
 use Test::NoWarnings;
 ;;
 use Data::Dump qw(pp);
 ;;
 note qq{perl version: $]};
 ;;
 my @TESTS = (
   [ undef                      , qq{}            ],
   [ qq{}                       , qq{}            ],
   [ qq{ }                      , qq{}            ],
   [ qq{\n}                     , qq{}            ],
   [ qq{\n\t}                   , qq{ }           ],
   [ qq{\n\t\x00}               , qq{ }           ],
   [ qq{\n\t \x00}              , qq{ }           ],
   [ qq{\n\t abc 123   xxx\n}   , qq{ abc123 xxx} ],
   [ qq{\nabc 123\a\b\fxxx\n\t }, qq{abc123 xxx } ],
   [ qq{abc 123\n\r xxx}        , qq{abc123 xxx}  ],
   );
 ;;
 note 'special case';
 is CollapseWhitespace(), '', 'no arguments';
 ;;
 note 'general cases';
 VECTOR:
 for my $ar_vector (@TESTS) {
   if (not ref $ar_vector) {
     note $ar_vector;
     next VECTOR;
     }
   ;;
   my ($str, $expected) = @$ar_vector;
   ;;
   is CollapseWhitespace($str), $expected,
      pp($str) . ' -> ' . pp($expected)
      ;
   }
 ;;
 done_testing;
 ;;
 exit;
 ;;
 sub CollapseWhitespace {
   my $s = shift;
   return '' unless defined $s;
   $s =~ s{ [\x00-\x20]+ }{ $+[0] - $-[0] == 1 ? '' : ' ' }xmsge;
   return $s;
   }
"
# perl version: 5.008009
# special case
ok 1 - no arguments
# general cases
ok 2 - undef -> ""
ok 3 - "" -> ""
ok 4 - " " -> ""
ok 5 - "\n" -> ""
ok 6 - "\n\t" -> " "
ok 7 - "\n\t\0" -> " "
ok 8 - "\n\t \0" -> " "
ok 9 - "\n\t abc 123   xxx\n" -> " abc123 xxx"
ok 10 - "\nabc 123\a\b\fxxx\n\t " -> "abc123 xxx "
ok 11 - "abc 123\n\r xxx" -> "abc123 xxx"
1..11
ok 12 - no warnings
1..12

[download]
If you have Perl version 5.14+, a slightly conciserer variation is: 
sub CollapseWhitespace {
  my $s = shift;
  return defined $s
    ? $s =~ s{ [\x00-\x20]+ }{ $+[0] - $-[0] == 1 ? '' : ' ' }xmsger
    : ''
    ;
  }

[download]
See the  s///  /r modifier in perlop. I leave it to you to Benchmark whether the  s///e version is actually faster than the for-loop version.

Give a man a fish:  <%-{-{-{-<

In reply to Re^2: Removing extra spaces by AnomalousMonk
in thread Removing extra spaces by rickoy

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