Complicated regexes are for the weak; Herr Doktor Conway's code is for the strong. I have to say that Text::Balanced worked a bit differently than I thought it would; I plan on fiddling with it a bit and reporting back on it.
This code is not particularly beautiful, but it does show a use of Text::Balanced.

use Text::Balanced "extract_delimited";
my $out;

my     $r = '
if (c=e)
     {                  // delete this curly brace
      call pgme;
      call pgmd;
      }                // delete this curly brace
     else
       {call pgmd;     // keep these curly braces
        call pgmc;}
if (c=e)
     {                  // delete this curly brace
      call pgme;
      call pgmd;
      }                // delete this curly brace
     else
       {call pgmd;     // keep these curly braces
        call pgmc;}

if (c=e)
     {                  // delete this curly brace
      call pgme;
      call pgmd;
      }                // delete this curly brace
     else
       {call pgmd;     // keep these curly braces
        call pgmc;}

if (c=e)
     {                  // delete this curly brace
      call pgme;
      call pgmd;
      }                // delete this curly brace
     else
       {call pgmd;     // keep these curly braces
        call pgmc;}
';

while (($e, $r, $s) = extract_delimited ($r, "{\"}", '(?s)[^{]*')){
    do {$out .= $r; last } unless  $e ;
    if ($s =~/if /) {
        $e =~s/[{]//;
        # make sure we don't nuke something in a quoted block.
        $e = reverse $e;
        $e =~s/[}]//;
        $e = reverse $e;
    };
    $out .= $s . $e;
}
print $out;
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