Win8 Strawberry 5.8.9.5 (32)  Wed 06/09/2021 15:18:22
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings

use Data::Dump qw(dd);

# my $line51 = 'foo';
for my $line51 (
    '', "\n", 'foo', 'foo, bist_mode, bar', 'foo, bist_mode bar',
    ) {

    dd '$line51', $line51;

    my @result = grep (/bist_mode/, $line51);
    dd 'A', \@result;

    my $b = join("_",@result);
    dd 'B', $b;

    $b =~ s/,([^,]*$)/$1/;
    dd 'C', $b;

    print "\n";

    }

^Z
("\$line51", "")
("A", [])
("B", "")
("C", "")

("\$line51", "\n")
("A", [])
("B", "")
("C", "")

("\$line51", "foo")
("A", [])
("B", "")
("C", "")

("\$line51", "foo, bist_mode, bar")
("A", ["foo, bist_mode, bar"])
("B", "foo, bist_mode, bar")
("C", "foo, bist_mode bar")

("\$line51", "foo, bist_mode bar")
("A", ["foo, bist_mode bar"])
("B", "foo, bist_mode bar")
("C", "foo bist_mode bar")