That regex doesn't match what you think it does. The . is special. Tip #6 from the Basic debugging checklist: YAPE::Regex::Explain:

----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  0                        '0'
----------------------------------------------------------------------
  .                        any character except \n
----------------------------------------------------------------------
  00000                    '00000'
----------------------------------------------------------------------
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You could use map to format certain elements of your array:

use warnings;
use strict;

my @cols = qw(3.456 8.903 1.223);
@cols = map { sprintf '%8.1f', $_ } @cols;
print "@cols\n";

__END__

     3.5      8.9      1.2
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Show some input and desired output.