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