Your /PATTERN/ appears to be whitespace (/s+).

Answers re split (and, as it happens, re join) can be found at perldoc -f split.

Working code (to your specs, except for using __DATA__ rather than a file as the data source):

#!/usr/bin/perl
use strict;
use warnings;

my($memAddr, $data, $ascii, @lines, $line);
# open(DATAFILE $filename)
# while(<DATAFILE>)
@lines = <DATA>;
    for $line(@lines) {
    chomp $line;
    ($memAddr, $data, $ascii) = split(/\s\s+/, $line);
    $data = join('', split(/\s/, $data));
    print "$memAddr $data $ascii \n";
}

__DATA__
-MEMADDR-  ----HEX REPRESENTATION----       ---ASCII---  
0000       21 74 63 5f 37 5f 31 5f 36        !tc_7_1_6
1234       0f bb ac 11 a6 82 ff 5f 27        !xy_8_1
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