...
my $var
...
sub initialize
{ ...
  $var = 'this string';
  ...
}

sub mysub
{
    # print STDERR "var = $var";
    print STDERR eval("...$var...");
}

BEGIN { initialize() }

##</code><code>##

Use of uninitialized value in concatenation (.) or string at (eval 40) line 1.