Here's one way

#! perl -sw
use strict;

sub immutable {
    print "immutable received args: [ @_ ]\n";
    print "Some more stuff\n";
}

## Save a copy of STDOUT
open SAVED, '>&=STDOUT';
close STDOUT;

## Open STDOUT to a variable (ramfile)(Requires 5.8.x)
open STDOUT, '>', \ my( $var ) or die $!;

## Call the sub
immutable( qw[ some args here ] );

## Close the ramfile
close STDOUT;

## Redirect STDOUT back to it's original place
open STDOUT, '>&=SAVED' or die $!;

## Discard the backup
close SAVED;

## Use the captured output
print "The variable \$var now contains:\n'$var'\n";
__END__
C:\test>junk2
The variable $var now contains:
'immutable received args: [ some args here ]
Some more stuff
'
[download]