You haven't shown us enough to demonstrate the problem.

use strict;
use warnings;

# $RtnVal = $ActionSubsM{$FormCmd}->( $FORM{'meetingid'} );

my %ActionSubsM;
my $FormCmd = "some_action";
my %FORM;
$FORM{meetingid} = "Hello world!\n";

$ActionSubsM{$FormCmd} = sub { return shift; };

my $RtnVal = $ActionSubsM{$FormCmd}->( $FORM{'meetingid'} );

print $RtnVal;
[download]

...outputs...

Hello world!
[download]

The code you are demonstrating is correct if the code you haven't demonstrated provides the necessary setup.

The best thing you could do is boil your problem down to a 10 to 20 line example that can be executed, and that will exhibit the problem you're facing. There's a strong chance that in doing so, you'll find the problem yourself anyway. And if not, you will have something you can post for us to look at. ;)