simple example to center text on a line

centertext.pl

#!/usr/bin/env perl -w

### 2016-06-01:davidm:simple program to center text on a line

use strict;
use warnings;
use POSIX;

#f1: string
#f2: total width
#f3: left border char
#f4: right border char
#f5: pad char

cent('t', 8, ':', ':', '.');
cent('te', 8, ':', ':', '.');
cent('tes', 8, ':', ':', '.');
cent('test', 8, ':', ':', '.');
cent('testt', 8, ':', ':', '.');
cent('testte', 8, ':', ':', '.');
cent('testtes', 8, ':', ':', '.');
cent('testtest', 8, ':', ':', '.');
cent('testtestt', 8, ':', ':', '.');

cent('t', 8, '', '', '.');
cent('te', 8, '', '', '.');
cent('tes', 8, '', '', '.');
cent('test', 8, '', '', '.');
cent('testt', 8, '', '', '.');
cent('testte', 8, '', '', '.');
cent('testtes', 8, '', '', '.');
cent('testtest', 8, '', '', '.');
cent('testtestt', 8, '', '', '.');

cent('t', 8, '>', '<' );
cent('te', 8, '>', '<' );
cent('tes', 8, '>', '<' );
cent('test', 8, '>', '<' );
cent('testt', 8, '>', '<' );
cent('testte', 8, '>', '<' );
cent('testtes', 8, '>', '<' );
cent('testtest', 8, '>', '<' );
cent('testtestt', 8, '>', '<' );

cent('t', 8);
cent('te', 8);
cent('tes', 8);
cent('test', 8);
cent('testt', 8);
cent('testte', 8);
cent('testtes', 8);
cent('testtest', 8);
cent('testtestt', 8);

sub cent {
    my $centertxt = shift;

    my $linewid = shift;

    my $lefttxt   = shift;
    my $righttxt  = shift;

    my $padchar = shift;

    $centertxt = ' ' if not defined $centertxt;
    $linewid   = 80  if not defined $linewid;
    $lefttxt   = ''  if not defined $lefttxt;
    $righttxt  = ''  if not defined $righttxt;
    $padchar   = ' ' if not defined $padchar;

    my $centerlen = length($centertxt);
    my $leftlen   = length($lefttxt);
    my $rightlen  = length($righttxt);

    my $topad = $linewid - $centerlen - $leftlen - $rightlen;

    if ($topad < 0 ) {
        $topad = 0;
    }

    $linewid = $linewid - $topad;

    my $leftpad = ( $padchar x floor($topad / 2 ) );
    my $rightpad = ( $padchar x ceil($topad / 2 ) );

    print "${lefttxt}${leftpad}${centertxt}${rightpad}${righttxt}\n"
}

Output looks like:

$ ./centertext.pl 
:..t...:
:..te..:
:.tes..:
:.test.:
:testt.:
:testte:
:testtes:
:testtest:
:testtestt:
...t....
...te...
..tes...
..test..
.testt..
.testte.
testtes.
testtest
testtestt
>  t   <
>  te  <
> tes  <
> test <
>testt <
>testte<
>testtes<
>testtest<
>testtestt<
   t    
   te   
  tes   
  test  
 testt  
 testte 
testtes 
testtest
testtestt
$