my $state;
open SESAME, "infile";
while (<SESAME>) {
 # Setting the "states" of the "state machine"
 if ( $_ =~ /FH/) { $state = "FH" }
 if ( $_ =~ /BH/) { $state = "BH"}
 .
 .
 # Do different things with the data depending on the
 # settings of the "state machine"
 if ( $state eq "FH" ) { # do this } 
 if ( $state eq "BH" ) { # do that }
 .
 .
}
close SESAME;