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.