Starting from "abcd| a a7-", you want to get "[a] [a7] [a]". I see 3 a's in the input, and 3 in the output, but I'm not sure what goes where. Except maybe the a7.

Can you be more clear? What have you got so far?

Further to kennethk's post: Perhaps also see I know what I mean. Why don't you?

I'd hoped OP would correct or clarify the (possible disparity?) between the order of the inputs and the order of the outputs in the original post -- but given the time that's passed since the replies above encouraged her to do so, here's one approach (with enough bad ideas such as some tender regexen and making all variables global that you should NOT cargo cult it; turn it in for homework; or use in any production environment):

#!/usr/bin/perl -w
use strict;
#1159495

my $input = 'abcd| a a7-';

my ( $item, @display, $display, $out, );

my @deSpaced = split / /, $input;

for $item(@deSpaced) {

    if ($item =~ /(a).*\|/ ) {           # match first part of $input?
        $out = '[' . $1 . ']';
        push @display, $out;
    }
    if ( $item =~ /(^a$)/ ) {               # match middle of $input?
          $out = '[' . $1 . ']';
        push @display, $out;

    } elsif ( $item =~ /(a[0-9]{1}).+/ ) {     # match last part of $i
+nput?
          $out = '[' . $1 . ']';
        push @display, $out;
    }
}

foreach my $saved(@display) {
    print $saved . ' ';
}

=head 


EXECUTION:  
C:\ PMonks\1159495.pl
[a] [a] [a7]
C:\

=cut
[download]

The code ignores ("leaves as an exercise for the Seeker") the re-ordering expressed in OP's desire to transform
"abcd| a a7-" into "a a7 a"
The last element with an "a" in the original is "a7" (ignoring the hyphen) But your output makes the bracketed "a7" appear before a final (bracketed) "a."