You can do it like this:

#!/usr/bin/perl

use strict;
use warnings;

my $usage ="$0 <fileA> <fileB>\n";

my $fileA = shift or die $usage ;
my $fileB = shift or die $usage;

open(FILEA, '<', $fileA) or die "could not open $fileA\n";
open(FILEB, '<', $fileB) or die "could not open $fileB\n";

my $file_a_content;

{
  local $/=undef; # unset input record separator to slurp all into a s
+tring
  $file_a_content = <FILEA> ;
  close FILEA;
}  

while (my $line = <FILEB> ) {
  $line=~s/SEARCHPATTERN/$file_a_content/;
  print $line;
}
close FILEB;
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This will print to STDOUT, so use it like this:

$ middle_replacer.pl fileA fileB > fileC
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