use strict;
use warnings;

my $string = shift;

if ( $string =~ /received 486 response/ ) {
    print "matched $string\n";
}
else {
    print "did not match $string\n";
}
[download]

Ran it with 'script.pl "received 486 response"' and it worked as intended. It does not match "received 485 response" or "received 480 response" or "received 487 response".

EDIT: Maybe you should post some code other that the pattern match, so we can see what is going on.

EDIT2: Here is another example:

my @array = (
    "received 480 response",
    "received 485 response",
    "received 486 response",
    "received 487 response"
);

for (@array) {
    if ( $_ =~ /received 486 response/ ) {
        print "matched $_\n";
    }
    else {
        print "did not match $_\n";
    }
}

________________________________________________

Output is:
F:\>script.pl
did not match received 480 response
did not match received 485 response
matched received 486 response
did not match received 487 response
[download]

So maybe what ever is inside your variable is indeed not what you think it is <.<