You want to use the \Q...\E escape around the variable in the LHS of your regex (or use quotemeta() on it) to stop Perl from treating it like a regex.

$from = '[hello]';
$to = '{goodbye}';
$string =~ s/\Q$from\E/$to/;

# or

$safe_from = quotemeta $from;
$string =~ s/$safe_from/$to/;
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