Your code is doing the following:

1. Find all the elements of @lines that begin with AUID (plus some optional whitespace) and store them in @headings,
2. take the first found element and do a split on ' ' (which has a special meaning as explained in its docs) and overwrite the contents of @headings with the result, and
3. shift the first element off @headings and discard it.

There are two problems with the oneliner you showed:

• grep in scalar context (as it is in the code you showed) will return the number of matches, not one of the matches, and
• shift removes the first element from an array and returns the element it removed, not the list minus the first element.

<update2> Eily's suggestion is nicest IMO:

my (undef, @headings) = split ' ', first {/^\s*AUID/} @lines;
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</update2>

You can pack it in a single line, here is one way, but it's kind of ugly. This uses (...)[0] to return the first return value of grep and splice to return the list without its first element:

my @headings = splice @{[
    split ' ', (grep /^\s*AUID/, @lines)[0] ]}, 1;
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Although I might recommend using first from List::Util to avoid the wasteful grepping of the entire array:

my @headings = splice @{[ split ' ', first {/^\s*AUID/} @lines ]}, 1;
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I still don't think this is very pretty because of the @{[]} trick I have to use to get splice to be happy. This one, using map and a regex, I like better:

my @headings = map {/(?!^\s*AUID)\s(\S+)/g} first {/^\s*AUID/} @lines;
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Update: map isn't really needed:

my @headings = (first {/^\s*AUID/} @lines)=~/(?!^\s*AUID)\s(\S+)/g;
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Rather than splice you can use undef to ignore unwanted elements. Like this: my (undef, @headings) = split ' ', first {/^\s*AUID/} @lines;

Going further (it's not related to the OP anymore) it works exactly as if there was one scalar in that place that was then ignored. So in the following example: (undef, $second, undef, $hash{fourth}, @rest, undef) = some_function() The first and third values are ignored, the second and fourth get in $second and $hash{fourth}, but @rest gets all the following values, and the last undef is useless. So it let's you ignore values counting from the left, but never couting from the right.

You're right of course, and that approach is probably much better in this case - thanks for pointing it out!

(That's what I get for posting hastily on a Sunday evening :-/ )

haukex has pointed out one problem with your use of shift. If this were the only problem, you could use splice instead. However, neither will work because both require an array (not a list). The difference is subtle, but the documentation of both have it right.
Bill

#!/usr/bin/perl

# https://perlmonks.org/?node_id=1226300

use strict;
use warnings;

my @lines = <<END =~ /.*\n/g;
other stuff
AUID \cIRA \cIDec \cILabel \cIV \cIB-V \cIComments 
more stuff
END

my @headings = (grep /^\s*AUID/, @lines)[0] =~ /\t(\S+)/g;

use Data::Dump 'dd'; dd \@headings;
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To do it without modules try:

my ( undef, @headings ) = split ' ', (grep /^\s*AUID/, @lines)[0];
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my ( undef, @headings ) = split ' ', shift @{[grep /^\s*AUID/, @lines]
+};
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Please clarify. :)

use strict;
use warnings;
use Data::Dump qw/pp dd/;

my @lines =<DATA>;

my @headings = grep /^\s*AUID/, @lines;
@headings = split ' ', $headings[0];
shift @headings;
pp @headings;

__DATA__
AUID \cIRA \cIDec \cILabel \cIV \cIB-V \cIComments
AUID \cIRA \cIDec \cILabel \cIV \cIB-V \cIComments
AUID \cIRA \cIDec \cILabel \cIV \cIB-V \cIComments
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(
  "\\cIRA",
  "\\cIDec",
  "\\cILabel",
  "\\cIV",
  "\\cIB-V",
  "\\cIComments",
)
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"\cI" is "\t" (tab), that's my guess as to what's meant...

My thanks to those who replied with solutions.

In response to the comment about the "AUID \cIRA \cIDec \cILabel \cIV \cIB-V \cIComments" imprecision, that was cut&pasted directly from a perl -d session and I'd hoped it would be recognized as such but you're right: I should have been more precise.