. means "any one character".
\. means "one period".
You keep using the wrong one.

You used (XXX)+ where you mean (XXX+).

Then there's using $2 when the regexp only has one capture.

You didn't create any records for interfaces with no descriptions (but your desired output indicates you want this).

$Recs is a more accurate name for $Rec.

The XXX, XXX if XXX syntax is unusual and therefore harder to read.

Recs has no reason for being a hash reference. A hash is sufficient.

Your indenting is inconsistent. Pick a style and stick with it.

use warnings was missing. It would have identified your problems here.

use strict;
use warnings;

my %Recs;
my $pc_name;

while (<DATA>) 
{
    if (/interface (\S+)$/) {
        $pc_name = $1;
        $Recs{$pc_name} = {};   # Create record.
        next;
    }

    if (/switchport description (.*)$/) {
        $Recs{$pc_name}{description} = $1;
        next;
    }
}

for $pc_name (keys %Recs)
{
    my $description = $Recs{$pc_name}{description};
    if (defined $description) {
        $description = "switchport description $description";
    } else {
        $description = "";
    }
    print("interface $pc_name = switchport description $description\n"
+);
}
[download]

or if you're not planning on capturing more than just the description:

use strict;
use warnings;

my %descriptions;
my $pc_name;

while (<DATA>) 
{
    if (/interface (\S+)$/) {
        $pc_name = $1;
        $descriptions{$pc_name} = undef;
        next;
    }

    if (/switchport description (.*)$/) {
        $descriptions{$pc_name} = $1;
        next;
    }
}

for $pc_name (keys %descriptions)
{
    my $description = $descriptions{$pc_name};
    if (defined $description) {
        $description = "switchport description $description";
    } else {
        $description = "";
    }
    print("interface $pc_name = switchport description $description\n"
+);
}
[download]

Update: Added missing paren.

Update: Added creation of records with no description.