sub numfix
{
   my $num  = shift;
   $num = sprintf("%.2f", $num);
   1 while $num =~ s/^([-+]?\d+)(\d{3})/$1,$2/;
   return $num;
}

foreach ( qw( 1000 1024 4.3 2.44 1024.44 ) ) {
    print( $_ );
    print( " => " );
    print( numfix( $_ ) );
    print( "\n" );
}