It's pretty straightforward in this case. The Javascript code in question is something along these lines:

var s='=b!isfg>(fybnqmfAfybnqmf/dpn(?fybnqmfAfybnqmf/dpn=0b?';var i;fo
+r (i=0;i<s.length;i++) document.write( String.fromCharCode(s.charCode
+At(i)-1));
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Even not knowing any Javascript it's not too difficult to tell what this does: it goes through the string one character at a time, and maps each character to the preceding character (in whatever character set Javascript uses by default).

You could do the same in Perl more or less verbatim (using the chr and ord functions), but instead of using a loop, it's perhaps more idiomatic to resort to split, map and join:

#!/usr/bin/perl

use strict;
use warnings;
use feature qw/say/;

my $s = '=b!isfg>(fybnqmfAfybnqmf/dpn(?fybnqmfAfybnqmf/dpn=0b?';

say join "", map { chr(ord($_) - 1) } split //, $s;
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This outputs:

$ perl 1119765.pl
<a href='example@example.com'>example@example.com</a>
$
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I would look at the MDN, but that requires that you already know enough Javascript.