use strict;
use warnings;

my @c;
my @a = qw ( server1 server2 server3 server5 );
my %a = map { $_ => 1 } @a;    # Solution 4

my $filename = 'file';
open my $fh, '<:crlf', $filename or die "$0: Can't open $filename $!";
while (defined(my $line = <$fh>)) {
    chomp $line;

    # Solution 1
    push @c, $line if not $line ~~ \@a;

    # Solution 2
    push @c, $line if not grep { $line eq $_ } @a;

    # Solution 3
    use List::Util qw(first);
    push @c, $line if not first { $_ eq $line } @a;

    # Solution 4
    push @c, $line if not exists $a{$line};
}
close $fh;

print "@c";