use strict;
use warnings;

use Config::Tiny;

my $DATA =<<EOT
localusername=me
ip=36.212.176.90
port=5060
remotename=you
ip=36.212.176.92
port=5060
transport=udp
EOT
;

# Create a config
my $Config = Config::Tiny->new();

# Open the config, replace with read and $DATA with filename
$Config = Config::Tiny->read_string( $DATA );
#$Config = Config::Tiny->read( $filename );

# see perldoc Config::Tiny
my $s = $Config->{_};

# I was to lazy to finish this:
my $INVITE = join(' ', 'INVITE sip:' . $s->{remotename} .'@'. $s->{ip}
+,
                       'SIP/2.0 Via:SIP/2.0/UDP', $s->{localusername} 
+. '@' . $s->{ip} .':5060',
                       'From:sam<sip:me@36.212.176.92>',
                       'To: you<sip:you@36.212.176.90>',
                 );
print $INVITE;
[download]

UPDATE: You probably want to add a [local] and a [remote] section in the config file. Then just access the variables with $r = Config->{remote} and build your INVITE line. See perldoc Config::Tiny.