In your example:

#!/perl/ -w

$pat=''; #thing to search for #line 3

while(){
print 'Enter the pattern';
chomp($pat =<stdin>);

print 'enter the string:';
chomp($_ = <stdin>); #10
if($_ =- m/$pat/g) {

print "true\n";
}

else{print "false\n";}

}
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There is only one "pattern match operator" and that is: m/$pat/g.    And the only problem with that is the use of the /g option, which in this case is superfluous.

Your problem is that the characters between $_ and m/$pat/g are not a valid perl operator.