while(my $line51 = <$ifh51>) {
   # my $line51 = $_ if /\bbist_mode\b/;
  @result = grep (/bist_mode/, $line51);
  @result1 =  grep (/mode_sel/, $line51);
  @result2 = grep (/mem_type_sel/, $line51);

    $b = join("_",@result);
    $b =~ s/,([^,]*$)/$1/;
    $c = join("_",@result1);
    $c =~ s/,([^,]*$)/$1/;
    $d = join("_",@result2);
    $d =~ s/,([^,]*$)/$1/;
    print $b;
    print $c;
    print $d;
  }

##</code><code>##

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")

##</code><code>##

Win8 Strawberry 5.8.9.5 (32)  Wed 06/09/2021 15:25:55
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings

use Data::Dump qw(dd);

my $data = <<'EOD';
bist_ctlr_clk_bistctlr,

bist_ctlr_rst_bistctlr_n,

bist_ctlr_serial_in_system,

bist_ctlr_serial_out_system,

bist_ctlr_shift_en_system,

bist_ctlr_global_bbad,

bist_ctlr_global_repairable,

bist_ctlr_bist_mode,

bist_ctlr_mem_type_sel,

bist_ctlr_mode_sel,

bist_ctlr_memsafe,

bist_ctlr_sif_reg_en_system,

bist_ctlr_sif_load_en_system,

bist_ctlr_sif_update_en_system,

bist_ctlr_mem_atpg_mode,

bist_ctlr_flash_data_select,

bist_ctlr_flash_repair_data,

bist_ctlr_bend,
EOD

open (my $ifh51, '<', \$data) or die $!;

my $rx_match = qr{ bist_mode | mode_sel | mem_type_sel }xms;
my @matched;

LINE51:
while (my $line51 = <$ifh51>) {

    # no further processing unless something matches.
    next LINE51 unless $line51 =~ $rx_match;

    # remove rightmost comma.
    $line51 =~ s{ , (?= [^,]* \Z) }{}xms;

    # save what was matched and processed.
    push @matched, $line51;

    }

dd \@matched;

^Z
[
  "bist_ctlr_bist_mode\n",
  "bist_ctlr_mem_type_sel\n",
  "bist_ctlr_mode_sel\n",
]