/\A\s*(.*?)\s*\z/

##</code><code>##

say for map { (/\A\s*(.*?)\s*\z/) } split /\|/, $str;

##</code><code>##

say for map { clean($_) } split /\|/, $str;
sub clean { ($_[0] =~ /\A\s*(.*?)\s*\z/)[0] }

##</code><code>##

$ perl -Mstrict -Mwarnings -E '
my $str = " \t item0 with leading whitespace |item1  | item2| item3 
  | extra item with embedded whitespace \t \n \n\t  
|item4|
 | itemN with trailing whitespace    \t
 ";
say "---- WITHOUT SUBROUTINE ---";
say ">$_<" for map { (/\A\s*(.*?)\s*\z/) } split /\|/, $str;
say "---- USING SUBROUTINE ---";
say "<$_>" for map { clean($_) } split /\|/, $str;
sub clean { ($_[0] =~ /\A\s*(.*?)\s*\z/)[0] }
'
---- WITHOUT SUBROUTINE ---
>item0 with leading whitespace<
>item1<
>item2<
>item3<
>extra item with embedded whitespace<
>item4<
><
>itemN with trailing whitespace<
---- USING SUBROUTINE ---
<item0 with leading whitespace>
<item1>
<item2>
<item3>
<extra item with embedded whitespace>
<item4>
<>
<itemN with trailing whitespace>