For a text string value, use :s for an optional value or =s for a required value:
#Getopt::Long::GetOptions('header=s' => \$header_string);
#!/usr/bin/perl -w
use Getopt::Long;
Getopt::Long::GetOptions(
'header=s' => \$header_string);
if ( defined( $header_string ) ) {
print "header flag set to $header_string\n";
}