You may be better off puting this data into a hash instead of using eval like this.

sub foo {
   my $format_string = shift;
   my %vars = (
      a => ...,
      b => ...,
      c => ...,
   );

   $format_string =~ s/(%(\w))/$vars{$1}/eg;
   return $format_string;
}
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That keeps the users out of your name space entirely, and drops the reliance on the 'eval'. This should be tons more efficient for you.