Here's some code that gets you close to what you want. It'll get you started...

use strict;
use warnings;

while (<DATA>) {
    if (/0x([[:xdigit:]]+)/) {
        my $hexstr = $1;
        my $word;
        while ($hexstr =~ /(..)/g) {$word .= chr hex $1}
        s/0x$hexstr/$word/;
    }
    print;
}

__DATA__
blah0x4445434C41524520405420blah blah
foo zoo 75858
[download]

prints out:

blahDECLARE @T lah blah
foo zoo 75858
[download]

You'll need to figure out how to deal with the 2nd "blah", whose 1st letter is a valid hex digit.