>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"}'

##</code><code>##

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;                                              
                                                                 
    }