No elegant alternative. As I showed in the benchmark, this works fine:

sub splt{split" ",reverse((split" ",(reverse$x),1)[0]),1;};
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To be honest, i all the years that I "do" Perl (since perl-4.016), I have never seen anyone using split to trim leading (or trailing) whitespace.

Had a reeeeeaaaaly vague recollection and the Perl Cookbook (Ch 1 section 19) does actually offer $_ = join(' ', split(' ')); as an alternative to these three substitutions:

s/^\s+//;
s/\s+$//;
s/\s+/ /g;
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to strip and canonicalize to single spaces; and does offer this trim sub:

sub trim {
    my @out = @_ ? @_ : $_;
    $_ = join(' ', split(' ')) for @out;
    return wantarray ? @out : "@out";
}
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Edit: That being said, I don't recall having seen this construct in the wild otherwise and had the vaguest of hunches that PC mentioned anything like this so I'd hardly call it a "common idiom" either.

The cake is a lie.
The cake is a lie.
The cake is a lie.

How often do you need to compress internal whitespace? Anyway. it is faster:

$ perl -MBenchmark=cmpthese -wE'my$x=join"    "=>"",("abc")x5,"";say"s
+ourc: |$x|";sub trim{join" ",split" ",$x};sub rgx{$x=~s/^\s+//r=~s/\s
++$//r=~s/\s\s+/ /gr};say "split: |",trim(),"|";say"regex: |",rgx(),"|
+";cmpthese(-2,{splt=>\&trim,rgx=>\&rgx})'
sourc: |    abc    abc    abc    abc    abc    |
split: |abc abc abc abc abc|
regex: |abc abc abc abc abc|
          Rate  rgx splt
rgx   506423/s   -- -71%
splt 1767763/s 249%   --
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Enjoy, Have FUN! H.Merijn

No elegant alternative. As I showed in the benchmark, this works fine:

sub splt{split" ",reverse((split" ",(reverse$x),1)[0]),1;};
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The most elegant approach to this problem is the use of tr function. The implementation below beats regex three times and can be made faster by trivial extension of tr function mentioned in my prev post (something like option 'x' -- stop the translation on the first symbol outside the set1 and return this position), which can be used instead of more general function index for searching single characters in the string and can made like rindex to be able to search in reverse direction too.

Looks like the solution for trim from the Cookbook mentioned by Fletch along with potentially deforming the string is slower then regex in my test(on my machine it took 3.56 sec real time).

Here is the "tr based" algorithm for trim:

time perl -e 'for (1..1000000) { $line=" aaa bbb ccc ddd eee fff "; $_=$line; $_=~tr/ /x/c; $start=index($_,'x'); $line=substr($line,$start,rindex($_,'x')-$start+1); }'

real 0m1.112s
user 0m1.076s
sys 0m0.031s

Can be made into a single statement making it slightly( ~7%) slower:

$line=substr($line,($start=index($_=$line=~tr/ /x/cr,'x')),rindex($_,'x')-$start+1);

# time perl -e 'for (1..1000000) { $line=" aaa bbb ccc ddd eee fff "; $line=substr($line,($start=index($_=$line=~tr/ /x/cr,'x')),rindex($_,'x')-$start+1); }' 
real 0m1.189s
user 0m1.154s
sys 0m0.015s

or

$line=substr($line,index($_=$line=~tr/ /x/cr,'x')),rindex($_,'x')-leng
+th($line)+1);
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