Hm, well...

If you just want to print these values out:

while (<DATA>) {
    chomp;
    print "\n$1 =" and next if /^(interface\s.+)/;
    print "$1" and next if /^\s+(switchport description\s.+)/;
}
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If getting them into a hash is the point:

my (%data, $last);
while (<DATA>) {
    #~ print STDERR $_;
    chomp;
    if (/^(interface\s.+)/) {
        $last = $1; 
        $data{$last} = '';
        next;
    }
    $data{$last}=$1 and next if /^\s+(switchport description\s.+)/;
}

##now print
for ( sort keys %data ) { print "$_ = $data{$_}\n" }
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The main problems you're having are illuminated in the posts above, so I won't belabor them. Just read perlre and perlretut again, and maybe pick up a copy of Mastering Regular Expressions (ISBN 0596002890).