Does anyone know why this occurs?

(in addition to ikegami's response above) - that's also because your $contents

my $contents =<<EOT;

####################
# GEOGRAPHY CONFIG #
####################

environment.type = eu
deployment.type = au

EOT
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contains " = au\n\n" after deployment.type. The regular expression engine sets its pointer just after the "type" in "deployment.type = au" - but the string doesn't finish at that point. That's why the position isn't reset.

But even if you'd correct that condition, the position at the end of the string would not be reset, since it is only reset if /g is exhausted, passing the end of the string. Consider:

use strict;
use warnings;

my $contents = "

####################
# GEOGRAPHY CONFIG #
####################

environment.type = eu
deployment.type = au";

if ( $contents !~ /deployment.type = (\w+)/gm)
{
    print "ERROR: deployment.type not found in contents\n";
}
else
{
    print "deployment.type matched\n";
}

$contents =~ /./g; # <-- this search passes end of string, and resets

if ( $contents !~ /environment.type = (\w+)/gm )
{
    print "ERROR: environment.type not found in contents\n";
}
else
{
    print "environment.type matched\n";
}
__END__

qwurx [shmem] 01:36 ~ > perl au.pl 
deployment.type matched
environment.type matched
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