You need "=" in substitution field. Replace "\1" with "$1" per warnings.

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 );
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OUTPUT:

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