sub _on_left {
    my ( $s ) = @_;
    my @words = split( /\s+/);
    my $word_level=3;
    return  $words[ scalar( @words ) - $word_level ];
}
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You say you want the third word from the left but you are accessing the third word from the right.    $words[ scalar( @words ) - $word_level ] should be $words[ $word_level - 1 ].

But you are not actually getting a word because split( /\s+/) is short for split( /\s+/, $_) and you don't have a value in $_ to split on.

So you want something like this:

sub _on_left {
    my ( $s ) = @_;
    my @words = split ' ', $s;
    my $word_level = 3;
    return  $words[ $word_level - 1 ];
}
[download]

sub order_line{  
    my @sorted_lines;
    foreach ( sort { _by_left( $a, $b ) } @lines ) { push @sorted_line
+s, $_ }
    print @sorted_lines;
}

sub _by_left {
    my ( $a, $b ) = @_;
    return lc( _on_left(  $a )) cmp lc( _on_left( $b )); 
    
}
[download]

You don't need a foreach loop, you can just do:

sub order_line{  
    my @sorted_lines = sort { _by_left( $a, $b ) } @lines;
    print @sorted_lines;
}
[download]

And instead of calling a subroutine in the sort block you could just use the subroutine name:

sub order_line{  
    my @sorted_lines = sort _by_left @lines;
    print @sorted_lines;
}

sub _by_left {
    return lc( _on_left(  $a )) cmp lc( _on_left( $b )); 
    
}
[download]