It allows for a simple variable to be used, instead of an expression including a variable: 
use warnings;
use strict;
my $foo = shift || 12;
my $i = 2;
print "Is \`$foo' a two-digit number ... ";
#my $rc = ($foo =~ /^[0-9]{($i + 1)}$/) ? 'yes' : 'no';
my $rc = ($foo =~ /^[0-9]{$i}$/) ? 'yes' : 'no';
#my $rc = ($foo =~ /^[0-9]{2}$/) ? 'yes' : 'no';
print $rc,"\n";

__END__

Is `12' a two-digit number ... yes

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Add re to your code to see what is happening under the hood: 

use re 'debug';

[download]

In reply to Re: Calculating variable {n} in regex match by toolic
in thread Calculating variable {n} in regex match by atreyu

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