NB: The  \1 \2 \3 notation is for regex back-references and is not kosher in a  s/// replacement string. warnings would have told you this had you enabled them. This (ab)use of back-references is a special dispensation of Perl, but should be avoided in favor of capture variables; see Variables related to regular expressions in perlvar.

c:\@Work\Perl\monks>perl -wMstrict -le
"use strict;
 use warnings;
 ;;
 my $line = 'OD44202,07/01/2015,08/19/2019,08/27/2019,1,07/01/2015,06/
+30/2019,2015-06-20 00:00:00';
 ;;
 $line =~ s/(\d{4})-(\d{2})-(\d{2})\s\d{2}:\d{2}:\d{2}/\2\/\3\/\1/g;
 print qq{>$line<};
"
\2 better written as $2 at -e line 1.
\3 better written as $3 at -e line 1.
\1 better written as $1 at -e line 1.
>OD44202,07/01/2015,08/19/2019,08/27/2019,1,07/01/2015,06/30/2019,06/2
+0/2015<
[download]

c:\@Work\Perl\monks>perl -wMstrict -le
"use strict;
 use warnings;
 ;;
 my $line = 'OD44202,07/01/2015,08/19/2019,08/27/2019,1,07/01/2015,06/
+30/2019,2015-06-20 00:00:00';
 ;;
 $line =~ s{(\d{4})-(\d{2})-(\d{2})\s\d{2}:\d{2}:\d{2}}{$2/$3/$1}g;
 print qq{>$line<};
"
>OD44202,07/01/2015,08/19/2019,08/27/2019,1,07/01/2015,06/30/2019,06/2
+0/2015<
[download]

Update: Your eyes will also thank you for some whitespace courtesy of the  /x modifier:
    $line =~ s{ (\d{4}) - (\d{2}) - (\d{2}) \s \d{2} : \d{2} : \d{2} }{$2/$3/$1}xg;
And is the  /g modifier really necessary?