use Data::Dumper;

my %vars;

# set initial variables
$vars{'INST'} = 'C';

open my $fh, "<", 'file' or die $!;
while (<$fh>) {
	chomp;
	m/^(\w+)=(.*)$/ or next;
	my ($var, $value) = ($1, $2);

	$value =~ s/\$([a-z]+)/ $vars{$1} /gie;
	$vars{$var} = $value;
}

print Dumper(\%vars);