#! perl -slw
use strict;

sub mutates {
    my $ref = defined wantarray ? \shift : \$_[ 0 ];
    $$ref =~ s[(?<=\b[^ ])([^ ]+)(?=[^ ]\b)][scalar reverse $1]ge;
    return $$ref if defined wantarray;
    return;
}

sub doSomething {
    print shift;
}

doSomething( mutates( 'antidisestablishmentarismania' ) );

my $var = 'The quick brown fox jumps over the lazy dog';
mutates( $var );
doSomething( $var );

__END__
c:\test>junk
ainamsiratnemhsilbatsesiditna
The qciuk bworn fox jpmus oevr the lzay dog