say "Found it" if $hash{$key}; 

##</code><code>##

say "Found it" if exists $hash{$key}; 

##</code><code>##

my %hash;
open my $INPUT, "<", $file_in or die "Cannot open $file_in $!"; # the die "..."  part is not needed if you use autodie.
while (my $line = <$INPUT>) {
    chomp $line;
    $hash{$line} = 1;  # or undef or anything you like for the value
}
close $INPUT;
# You can use %hash for lookup now