Win8 Strawberry 5.8.9.5 (32)  Thu 07/28/2022 11:22:44
C:\@Work\Perl\monks
>perl

use strict;
use warnings;

use Data::Dump qw(dd);

use constant FIELDNAME => 'fieldname';

open my $fh, '<', \<<END or die;
no valid header present
value1,value2
END

my $got_header = do { local $/ = FIELDNAME;  <$fh>; };
dd $got_header;  # for debug
$got_header =~ m{ \Q${ \FIELDNAME }\E \z }xms or die "no header";

my $header_line = FIELDNAME . <$fh>; # complete the line
# and so on...

^Z
"no valid header present\nvalue1,value2\n"
no header at - line 16, <$fh> line 1.