I agree that a two-step approach (i.e., match for name_a, then delete unwanted params) is best. However, here's an 'advanced' (read: unnecessarily complex) way that doesn't use the  /e modifier: 

>perl -wMstrict -le
"my @txt = (
   '{name_b param_v=\"wh\"}',
   '{name_a param_x=\"abc\" param_a=\"fsd\" param_y=\"def\"}',
   '{name_z param_sd=\"zka\" param_s=\"df\"}',
   '{name_a param_y=\"wtf\" param_z=\"kro\" param_c=\"ptz\" param_ch=\
+"www\"}',
   '{name_a param_sd=\"zka\" param_y=\"wtf\"}',
   );
 ;;
 my $not_p_xy     = qr{ (?! param_ [xy] \b)             }xms;
 my $not_sp_param = qr{ (?! \s+ $not_p_xy param_ \w+) . }xms;
 ;;
 for my $s (@txt) {
   print qq{'$s'};
   $s =~ s(
     (?: \G (?<! \A) | \A {name_a) $not_sp_param* \K
     \s+ $not_p_xy \w+ = \" [^^\"]* \"
     )
     ()xmsg;
   print qq{'$s' \n};
   }
"
'{name_b param_v="wh"}'
'{name_b param_v="wh"}'

'{name_a param_x="abc" param_a="fsd" param_y="def"}'
'{name_a param_x="abc" param_y="def"}'

'{name_z param_sd="zka" param_s="df"}'
'{name_z param_sd="zka" param_s="df"}'

'{name_a param_y="wtf" param_z="kro" param_c="ptz" param_ch="www"}'
'{name_a param_y="wtf"}'

'{name_a param_sd="zka" param_y="wtf"}'
'{name_a param_y="wtf"}'

[download]

Updates:

  1. Removed extraneous  (?: ) grouping around  \K expression above. Example output unchanged.
  2. I agree with Re^2: RegExp: pos management in global substitution regarding efficiency, and have further simplified (IMO) to: 
    sub xform {                                    
                                                                 
    my ($string,                                                 
        ) = @_;                                                  
                                                                 
    my $xy        = qr{     [xy]    }xmso;                       
    my $val       = qr{ = ' [^']* ' }xmso;                       
    my $param_    = qr{ \s+ param_  }xmso;                       
    my $param_xy  = qr{ $param_ $xy $val }xmso;                  
    my $param_any = qr{ $param_ \w+ $val }xmso;                  
                                                                 
    $string =~ s{                                                
        (?: \G (?<! ^) | ^ \{ name_a) $param_xy*+ \K $param_any
        }                                                        
        ''xmsg;                                                  
                                                                 
    return $string;                                              
                                                                 
    }

    [download]
    Update: Note that I'm using ' (single-quote) instead of " (double-quote) as the parameter value delimiter in my code and testing. This uses 5.10+ regex features. This has withstood everything I have thrown at it (including the multi-line example of the OP!), and I think it's my final answer. (This even qualifies as perhaps not unnecessarily complex!)

In reply to Re: RegExp: pos management in global substitution by AnomalousMonk
in thread RegExp: pos management in global substitution by OlegG

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