Use one of the various templating modules from CPAN:

• Text::Template

  use strict;
  use warnings;
  use Text::Template;
  
  my $template = Text::Template->new(
      TYPE   => 'STRING',
      SOURCE => 'fred has {$color} eyes'
  );
  
  our $color = 'blue';
  print $template->fill_in;
  [download]

• HTML::Template

  use strict;
  use warnings;
  use HTML::Template;
  
  my $data = 'fred has <tmpl_var color> eyes';
  
  my $template = HTML::Template->new(
      scalarref => \$data,
  );
  
  $template->param(
      color => 'brown',
  );
  
  print $template->output;
  [download]

• Template

  use strict;
  use warnings;
  use Template;
  
  my $data = 'fred has [% color %] eyes';
  my $vars = {
      color => 'blue',
  };
  
  my $tt = Template->new;
  $tt->process(\$data, $vars) || die $tt->error;
  [download]

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)