do { local $/ = "fieldname"; <$fh> };  # read through "fieldname"
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The local $/ = "fieldname"; statement sets the $/ input record separator special variable (see perlvar) to the literal 'fieldname' string. This sets "paragraph" read mode: (no: see Update below) | This causes <$fh> to read the input stream from the beginning of the file (in this particular case) until the end of the first point at which the 'fieldname' string is encountered. Since the field names are apparently unambiguously known, this reads (almost) all the way through the first field name. The $/ variable is assigned local-ly in a do-block, so it returns to its previous value (the "\n" default in this case) at the end of the block.

my $header_line = "fieldname" . <$fh>; # complete the line
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Since we (apparently) know the header line begins with 'fieldname', assign $header_line this initial value and complete reading the line with another <$fh>. This reads through the end of the line because $/ has restored to its original newline value.

Update:

This sets "paragraph" read mode: ...

In normal read mode, a file is read until just after the sequence of one or more characters in the $/ special variable is encountered (and including that sequence), or until the end of file if the $/ sequence is never encountered. Usually, $/ is a single "\n" (newline) character, but it can be any non-empty string. Whatever non-zero-length sequence of characters it may be, this is normal read mode.

use v5.12;
use warnings;
use Data::Dump qw/pp dd/;

say pp $/;                              # show default

say my $x =
  "HEADER\n" x 3 .
  "fieldname: BLA BLA\n" .
  join $/, 1..5;

open my $fh, '<', \$x;

# ignore anything prior to "fieldname"
{ local $/ = "fieldname:"; <$fh> };

say pp $/;                              # back to default

say "-" x 10;

say "fieldname:" . <$fh>;               # till end of line
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"\n"
HEADER
HEADER
HEADER
fieldname: BLA BLA
1
2
3
4
5
"\n"
----------
fieldname: BLA BLA
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It occurs to me that there might be a use for a do-block. If no 'fieldname' string is present in the file, <$fh> will read to the end of the file. This will be a valid read. If the data of the read is returned as from a do-block, it can be tested to determine if a header was actually present. (The data could be returned from a "naked" block, but using a do-block is neater IMHO.)

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.
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---

Give a man a fish:  <%-{-{-{-<

well, yes if you want to capture the first 'readline', a do block is better, because of all the scoping.

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery}