I don't understand the logic behind the substitutions.
This does what you said, though not in a very flexible way, I'm afraid...

#!/usr/bin/perl -w
use strict;

while(<DATA>){
  if( m!\+ vss:G vssa:I! ){
    print "+ vss:I vssa:G\n";
  } elsif( m!test2block! ){
    print "XI48 a b c vdd vcc vss vssa / cd_acore_porbuf\n";
  } else {
    print;
  }
}
__DATA__
.SUBCKT testblock a b c vdd vcc vss vssa
*.PININFO a:I b:I c:I vdd:I vcc:I 
+ vss:G vssa:I
RR0 a vss 9.99967K $[PO]
XI48 a b c vdd vcc vss vssa / test2block
.ENDS
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Could you add a little more context to the question?

s!!password!;y?sordid?binger?;
y.paw.mrk.;;print chr 0x5b ;;;
print;print chr(0x5b+0x2);;;;;
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