my $c = '+000000';
print 'contains' if $c =~ /\+\d\d\d\d\d\d/;
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Note that the s flag is for substitution. What you want is a match, so the s is left out. Also, as people have stated above, $a is a reserved variable, and while you can use it for this, you probably shouldn't. Also, as stated above, your regex can be shortened:

my $c = '+000000';
print 'contains' if $c =~ /\+\d{6}/;
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And if you need to match exactly:

my $c = '+000000';
print 'contains' if $c =~ /^\+\d{6}$/;
# or...
print 'contains' if $c eq '+'.'0'x6;
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