I'll assume the list is sorted. If not, either sort it or use List::Util's min/max functions to set up the min( @list ) .. max( @list ) range.

my @list = ( 41888, 41889, 41890, 41892, 41895 );

my @missing = do {
    my %missing;
    undef @missing{ $list[ 0 ] .. $list[ -1 ] };
    delete @missing{ @list };
    keys %missing;
};
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That's the simplest way; it takes some memory and I'm not sure about its efficiency, so use with very large lists is not necessarily advised.

Update: another option, assuming that the list really is sorted (can't rely on min/max here):

my @missing;
{
    my $i = 0;
    for ( $list[ 0 ] .. $list[ -1 ] ) {
        ++$i, next if $_ == $list[ $i ];
        push @missing, $_;
    }
}
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Update: and just because I had to, the same thing in functional lingo:

my @missing = do {
    my $i = 0;
    grep { $_ == $list[ $i ] ? ++$i && 0 : 1 } $list[ 0 ] .. $list[ -1
+ ];
};
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