Assuming integers, and given your single and very specific test case, I think ++ikegami's second solution is probably the best.

Here's a solution that's less compact and less efficient:

if ($x =~ /^(?:${\join "|", $y-1 .. $y+1})$/)
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My test:

$ perl -E 'my $y = 3; say "$_: ", /^(?:${\join "|", $y-1 .. $y+1})$/ ?
+ "YES" : "NO" for 1..5'
1: NO
2: YES
3: YES
4: YES
5: NO
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Having said that, given another test case with a larger range not centered exactly around $y, this technique may be more appealing. Consider

if ($x==$y or $x==$y-1 or $x==$y-2 or $x==$y-3 or $x==$y+1)
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vs.

if ($x =~ /^(?:${\join "|", $y-3 .. $y+1})$/)
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My test:

$ perl -E 'my $y = 3; say "$_: ", /^(?:${\join "|", $y-3 .. $y+1})$/ ?
+ "YES" : "NO" for -1..5'
-1: NO
0: YES
1: YES
2: YES
3: YES
4: YES
5: NO
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And those hard-coded numbers (in $y-3 and $y+1) could be variables: perhaps, $min_below_y and $max_above_y.

So, as I said, in your very specific example, this technique wouldn't be the best choice; however, in a more general scenario, it could be a better choice.