It works, but using a backreference \1 in place of a capture variable $1 in the replacement string is Frowned Upon and will earn you a warning:

Win8 Strawberry 5.8.9.5 (32)  Thu 01/28/2021 14:55:21
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings

use Data::Dump qw(dd);

my $s = "parameter=TO_DATE('1900-01-01','YYYY-MM-DD')\n";
dd 'before:', $s;

$s =~ s/=(.*)/="\1"/;
\1 better written as $1 at - line 7.
dd 'after:', $s;

^Z
("before:", "parameter=TO_DATE('1900-01-01','YYYY-MM-DD')\n")
(
  "after:",
  "parameter=\"TO_DATE('1900-01-01','YYYY-MM-DD')\"\n",
)
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use strict;
use warnings;
use Test::Simple tests=>1;

my $input_string = qq(parameter=TO_DATE('1900-01-01','YYYY-MM-DD')\n);
my $desired_output = qq(parameter="TO_DATE('1900-01-01','YYYY-MM-DD')"
+\n);

my $output = $input_string =~ s/=(.*)/="$1"/r;

ok( $output eq $desired_output, $output );
[download]

OUTPUT:

1..1
ok 1 - parameter="TO_DATE('1900-01-01','YYYY-MM-DD')"
#
[download]

You need "=" in substitution field.

And I had it. What's your point?

OOPS, Perhaps I need new glasses!
Bill