What do you mean by "good way"? There are allways more ways to do it...

If the above code works, it works. Not that there are things to improve; for example, you fill an array, which you later join. Why not join the lines directly?A (slightly rewriten) version of your code could be:

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

use POSIX;
use Win32::OLE;

open my $source, "<", "//reports/test.txt"
    or die "cannot open file: $!\n";

my $ies_details;

while( <$source> ) {
    next unless substr $source,11,4  =~ /\d{4}/;
    next unless substr $source,79,10 =~ /\d{10}/;

    $ies_details .= $_;
}

my $Outlook  = Win32::OLE->GetActiveObject('Outlook.Application')
            || Win32::OLE->new('Outlook.Application');

my $mailitem = $Outlook->CreateItem(0)
    or die "Outlook is not running, cannot send mail.\n";

my $procdate = POSIX::strftime('%m/%d/%Y %H:%M', localtime);

$mailitem->{'To'}      = 'test@yahoo.com';
$mailitem->{'Cc'}      = 'test@test.com';
$mailitem->{'Subject'} = 'Summary Report ' . $procdate;
$mailitem->{'Body'}    = "Below is a summary\n\n"
                       . "-----------------------------------------\n\
+n"
                       . $ies_details;

$mailitem->{'Importance'} = 0; # 2=high, 1=normal, 0=low

$mailitem->Send;
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